From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from ec2-52-27-115-49.us-west-2.compute.amazonaws.com ([52.27.115.49]:40073 "EHLO osg.samsung.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932625AbdDENXm (ORCPT ); Wed, 5 Apr 2017 09:23:42 -0400 From: Mauro Carvalho Chehab To: Linux Media Mailing List , Linux Doc Mailing List , Jonathan Corbet Cc: Mauro Carvalho Chehab , Mauro Carvalho Chehab , Greg Kroah-Hartman , linux-usb@vger.kernel.org, Jani Nikula , Takashi Iwai , Markus Heiser , Jonathan Cameron , Lukas Wunner , Daniel Vetter , "Rafael J. Wysocki" , Oliver Neukum Subject: [PATCH v2 03/21] docs-rst: convert usb docbooks to ReST Date: Wed, 5 Apr 2017 10:22:57 -0300 Message-Id: <2ef7194e2199bcd01b81647380f5b79e6737ab96.1491398120.git.mchehab@s-opensource.com> In-Reply-To: References: MIME-Version: 1.0 In-Reply-To: References: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-media-owner@vger.kernel.org List-ID: As we're moving out of DocBook, let's convert the remaining USB docbooks to ReST. The transformation itself on this patch is a no-brainer conversion using pandoc via this script: Documentation/sphinx/tmplcvt Signed-off-by: Mauro Carvalho Chehab --- Documentation/DocBook/Makefile | 7 +- Documentation/DocBook/gadget.tmpl | 793 ------------------- Documentation/DocBook/writing_musb_glue_layer.tmpl | 873 --------------------- Documentation/DocBook/writing_usb_driver.tmpl | 412 ---------- Documentation/driver-api/index.rst | 2 +- Documentation/driver-api/usb/gadget.rst | 533 +++++++++++++ Documentation/driver-api/usb/index.rst | 17 + Documentation/driver-api/{ => usb}/usb.rst | 168 ++-- .../driver-api/usb/writing_musb_glue_layer.rst | 737 +++++++++++++++++ .../driver-api/usb/writing_usb_driver.rst | 344 ++++++++ 10 files changed, 1719 insertions(+), 2167 deletions(-) delete mode 100644 Documentation/DocBook/gadget.tmpl delete mode 100644 Documentation/DocBook/writing_musb_glue_layer.tmpl delete mode 100644 Documentation/DocBook/writing_usb_driver.tmpl create mode 100644 Documentation/driver-api/usb/gadget.rst create mode 100644 Documentation/driver-api/usb/index.rst rename Documentation/driver-api/{ => usb}/usb.rst (87%) create mode 100644 Documentation/driver-api/usb/writing_musb_glue_layer.rst create mode 100644 Documentation/driver-api/usb/writing_usb_driver.rst diff --git a/Documentation/DocBook/Makefile b/Documentation/DocBook/Makefile index 4a81e7a78e23..13056d40e11b 100644 --- a/Documentation/DocBook/Makefile +++ b/Documentation/DocBook/Makefile @@ -8,12 +8,11 @@ DOCBOOKS := z8530book.xml \ kernel-hacking.xml kernel-locking.xml \ - writing_usb_driver.xml networking.xml \ + networking.xml \ filesystems.xml lsm.xml kgdb.xml \ - gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \ + libata.xml mtdnand.xml librs.xml rapidio.xml \ s390-drivers.xml scsi.xml \ - sh.xml w1.xml \ - writing_musb_glue_layer.xml + sh.xml w1.xml ifeq ($(DOCBOOKS),) diff --git a/Documentation/DocBook/gadget.tmpl b/Documentation/DocBook/gadget.tmpl deleted file mode 100644 index 641629221176..000000000000 --- a/Documentation/DocBook/gadget.tmpl +++ /dev/null @@ -1,793 +0,0 @@ - - - - - - USB Gadget API for Linux - 20 August 2004 - 20 August 2004 - - - - This documentation is free software; you can redistribute - it and/or modify it under the terms of the GNU General Public - License as published by the Free Software Foundation; either - version 2 of the License, or (at your option) any later - version. - - - - This program is distributed in the hope that it will be - useful, but WITHOUT ANY WARRANTY; without even the implied - warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. - See the GNU General Public License for more details. - - - - You should have received a copy of the GNU General Public - License along with this program; if not, write to the Free - Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, - MA 02111-1307 USA - - - - For more details see the file COPYING in the source - distribution of Linux. - - - - 2003-2004 - David Brownell - - - - David - Brownell - -
dbrownell@users.sourceforge.net
- - - - - - -Introduction - -This document presents a Linux-USB "Gadget" -kernel mode -API, for use within peripherals and other USB devices -that embed Linux. -It provides an overview of the API structure, -and shows how that fits into a system development project. -This is the first such API released on Linux to address -a number of important problems, including: - - - Supports USB 2.0, for high speed devices which - can stream data at several dozen megabytes per second. - - Handles devices with dozens of endpoints just as - well as ones with just two fixed-function ones. Gadget drivers - can be written so they're easy to port to new hardware. - - Flexible enough to expose more complex USB device - capabilities such as multiple configurations, multiple interfaces, - composite devices, - and alternate interface settings. - - USB "On-The-Go" (OTG) support, in conjunction - with updates to the Linux-USB host side. - - Sharing data structures and API models with the - Linux-USB host side API. This helps the OTG support, and - looks forward to more-symmetric frameworks (where the same - I/O model is used by both host and device side drivers). - - Minimalist, so it's easier to support new device - controller hardware. I/O processing doesn't imply large - demands for memory or CPU resources. - - - - -Most Linux developers will not be able to use this API, since they -have USB "host" hardware in a PC, workstation, or server. -Linux users with embedded systems are more likely to -have USB peripheral hardware. -To distinguish drivers running inside such hardware from the -more familiar Linux "USB device drivers", -which are host side proxies for the real USB devices, -a different term is used: -the drivers inside the peripherals are "USB gadget drivers". -In USB protocol interactions, the device driver is the master -(or "client driver") -and the gadget driver is the slave (or "function driver"). - - -The gadget API resembles the host side Linux-USB API in that both -use queues of request objects to package I/O buffers, and those requests -may be submitted or canceled. -They share common definitions for the standard USB -Chapter 9 messages, structures, and constants. -Also, both APIs bind and unbind drivers to devices. -The APIs differ in detail, since the host side's current -URB framework exposes a number of implementation details -and assumptions that are inappropriate for a gadget API. -While the model for control transfers and configuration -management is necessarily different (one side is a hardware-neutral master, -the other is a hardware-aware slave), the endpoint I/0 API used here -should also be usable for an overhead-reduced host side API. - - - - -Structure of Gadget Drivers - -A system running inside a USB peripheral -normally has at least three layers inside the kernel to handle -USB protocol processing, and may have additional layers in -user space code. -The "gadget" API is used by the middle layer to interact -with the lowest level (which directly handles hardware). - - -In Linux, from the bottom up, these layers are: - - - - - - USB Controller Driver - - - This is the lowest software level. - It is the only layer that talks to hardware, - through registers, fifos, dma, irqs, and the like. - The <linux/usb/gadget.h> API abstracts - the peripheral controller endpoint hardware. - That hardware is exposed through endpoint objects, which accept - streams of IN/OUT buffers, and through callbacks that interact - with gadget drivers. - Since normal USB devices only have one upstream - port, they only have one of these drivers. - The controller driver can support any number of different - gadget drivers, but only one of them can be used at a time. - - - Examples of such controller hardware include - the PCI-based NetChip 2280 USB 2.0 high speed controller, - the SA-11x0 or PXA-25x UDC (found within many PDAs), - and a variety of other products. - - - - - - Gadget Driver - - - The lower boundary of this driver implements hardware-neutral - USB functions, using calls to the controller driver. - Because such hardware varies widely in capabilities and restrictions, - and is used in embedded environments where space is at a premium, - the gadget driver is often configured at compile time - to work with endpoints supported by one particular controller. - Gadget drivers may be portable to several different controllers, - using conditional compilation. - (Recent kernels substantially simplify the work involved in - supporting new hardware, by autoconfiguring - endpoints automatically for many bulk-oriented drivers.) - Gadget driver responsibilities include: - - - handling setup requests (ep0 protocol responses) - possibly including class-specific functionality - - returning configuration and string descriptors - - (re)setting configurations and interface - altsettings, including enabling and configuring endpoints - - handling life cycle events, such as managing - bindings to hardware, - USB suspend/resume, remote wakeup, - and disconnection from the USB host. - - managing IN and OUT transfers on all currently - enabled endpoints - - - - - Such drivers may be modules of proprietary code, although - that approach is discouraged in the Linux community. - - - - - Upper Level - - - Most gadget drivers have an upper boundary that connects - to some Linux driver or framework in Linux. - Through that boundary flows the data which the gadget driver - produces and/or consumes through protocol transfers over USB. - Examples include: - - - user mode code, using generic (gadgetfs) - or application specific files in - /dev - - networking subsystem (for network gadgets, - like the CDC Ethernet Model gadget driver) - - data capture drivers, perhaps video4Linux or - a scanner driver; or test and measurement hardware. - - input subsystem (for HID gadgets) - - sound subsystem (for audio gadgets) - - file system (for PTP gadgets) - - block i/o subsystem (for usb-storage gadgets) - - ... and more - - - - - Additional Layers - - - Other layers may exist. - These could include kernel layers, such as network protocol stacks, - as well as user mode applications building on standard POSIX - system call APIs such as - open(), close(), - read() and write(). - On newer systems, POSIX Async I/O calls may be an option. - Such user mode code will not necessarily be subject to - the GNU General Public License (GPL). - - - - - - -OTG-capable systems will also need to include a standard Linux-USB -host side stack, -with usbcore, -one or more Host Controller Drivers (HCDs), -USB Device Drivers to support -the OTG "Targeted Peripheral List", -and so forth. -There will also be an OTG Controller Driver, -which is visible to gadget and device driver developers only indirectly. -That helps the host and device side USB controllers implement the -two new OTG protocols (HNP and SRP). -Roles switch (host to peripheral, or vice versa) using HNP -during USB suspend processing, and SRP can be viewed as a -more battery-friendly kind of device wakeup protocol. - - -Over time, reusable utilities are evolving to help make some -gadget driver tasks simpler. -For example, building configuration descriptors from vectors of -descriptors for the configurations interfaces and endpoints is -now automated, and many drivers now use autoconfiguration to -choose hardware endpoints and initialize their descriptors. - -A potential example of particular interest -is code implementing standard USB-IF protocols for -HID, networking, storage, or audio classes. -Some developers are interested in KDB or KGDB hooks, to let -target hardware be remotely debugged. -Most such USB protocol code doesn't need to be hardware-specific, -any more than network protocols like X11, HTTP, or NFS are. -Such gadget-side interface drivers should eventually be combined, -to implement composite devices. - - - - - -Kernel Mode Gadget API - -Gadget drivers declare themselves through a -struct usb_gadget_driver, which is responsible for -most parts of enumeration for a struct usb_gadget. -The response to a set_configuration usually involves -enabling one or more of the struct usb_ep objects -exposed by the gadget, and submitting one or more -struct usb_request buffers to transfer data. -Understand those four data types, and their operations, and -you will understand how this API works. - - -Incomplete Data Type Descriptions - -This documentation was prepared using the standard Linux -kernel docproc tool, which turns text -and in-code comments into SGML DocBook and then into usable -formats such as HTML or PDF. -Other than the "Chapter 9" data types, most of the significant -data types and functions are described here. - - -However, docproc does not understand all the C constructs -that are used, so some relevant information is likely omitted from -what you are reading. -One example of such information is endpoint autoconfiguration. -You'll have to read the header file, and use example source -code (such as that for "Gadget Zero"), to fully understand the API. - - -The part of the API implementing some basic -driver capabilities is specific to the version of the -Linux kernel that's in use. -The 2.6 kernel includes a driver model -framework that has no analogue on earlier kernels; -so those parts of the gadget API are not fully portable. -(They are implemented on 2.4 kernels, but in a different way.) -The driver model state is another part of this API that is -ignored by the kerneldoc tools. - - - -The core API does not expose -every possible hardware feature, only the most widely available ones. -There are significant hardware features, such as device-to-device DMA -(without temporary storage in a memory buffer) -that would be added using hardware-specific APIs. - - -This API allows drivers to use conditional compilation to handle -endpoint capabilities of different hardware, but doesn't require that. -Hardware tends to have arbitrary restrictions, relating to -transfer types, addressing, packet sizes, buffering, and availability. -As a rule, such differences only matter for "endpoint zero" logic -that handles device configuration and management. -The API supports limited run-time -detection of capabilities, through naming conventions for endpoints. -Many drivers will be able to at least partially autoconfigure -themselves. -In particular, driver init sections will often have endpoint -autoconfiguration logic that scans the hardware's list of endpoints -to find ones matching the driver requirements -(relying on those conventions), to eliminate some of the most -common reasons for conditional compilation. - - -Like the Linux-USB host side API, this API exposes -the "chunky" nature of USB messages: I/O requests are in terms -of one or more "packets", and packet boundaries are visible to drivers. -Compared to RS-232 serial protocols, USB resembles -synchronous protocols like HDLC -(N bytes per frame, multipoint addressing, host as the primary -station and devices as secondary stations) -more than asynchronous ones -(tty style: 8 data bits per frame, no parity, one stop bit). -So for example the controller drivers won't buffer -two single byte writes into a single two-byte USB IN packet, -although gadget drivers may do so when they implement -protocols where packet boundaries (and "short packets") -are not significant. - - -Driver Life Cycle - -Gadget drivers make endpoint I/O requests to hardware without -needing to know many details of the hardware, but driver -setup/configuration code needs to handle some differences. -Use the API like this: - - - - -Register a driver for the particular device side -usb controller hardware, -such as the net2280 on PCI (USB 2.0), -sa11x0 or pxa25x as found in Linux PDAs, -and so on. -At this point the device is logically in the USB ch9 initial state -("attached"), drawing no power and not usable -(since it does not yet support enumeration). -Any host should not see the device, since it's not -activated the data line pullup used by the host to -detect a device, even if VBUS power is available. - - -Register a gadget driver that implements some higher level -device function. That will then bind() to a usb_gadget, which -activates the data line pullup sometime after detecting VBUS. - - -The hardware driver can now start enumerating. -The steps it handles are to accept USB power and set_address requests. -Other steps are handled by the gadget driver. -If the gadget driver module is unloaded before the host starts to -enumerate, steps before step 7 are skipped. - - -The gadget driver's setup() call returns usb descriptors, -based both on what the bus interface hardware provides and on the -functionality being implemented. -That can involve alternate settings or configurations, -unless the hardware prevents such operation. -For OTG devices, each configuration descriptor includes -an OTG descriptor. - - -The gadget driver handles the last step of enumeration, -when the USB host issues a set_configuration call. -It enables all endpoints used in that configuration, -with all interfaces in their default settings. -That involves using a list of the hardware's endpoints, enabling each -endpoint according to its descriptor. -It may also involve using usb_gadget_vbus_draw -to let more power be drawn from VBUS, as allowed by that configuration. -For OTG devices, setting a configuration may also involve reporting -HNP capabilities through a user interface. - - -Do real work and perform data transfers, possibly involving -changes to interface settings or switching to new configurations, until the -device is disconnect()ed from the host. -Queue any number of transfer requests to each endpoint. -It may be suspended and resumed several times before being disconnected. -On disconnect, the drivers go back to step 3 (above). - - -When the gadget driver module is being unloaded, -the driver unbind() callback is issued. That lets the controller -driver be unloaded. - - - - -Drivers will normally be arranged so that just loading the -gadget driver module (or statically linking it into a Linux kernel) -allows the peripheral device to be enumerated, but some drivers -will defer enumeration until some higher level component (like -a user mode daemon) enables it. -Note that at this lowest level there are no policies about how -ep0 configuration logic is implemented, -except that it should obey USB specifications. -Such issues are in the domain of gadget drivers, -including knowing about implementation constraints -imposed by some USB controllers -or understanding that composite devices might happen to -be built by integrating reusable components. - - -Note that the lifecycle above can be slightly different -for OTG devices. -Other than providing an additional OTG descriptor in each -configuration, only the HNP-related differences are particularly -visible to driver code. -They involve reporting requirements during the SET_CONFIGURATION -request, and the option to invoke HNP during some suspend callbacks. -Also, SRP changes the semantics of -usb_gadget_wakeup -slightly. - - - - -USB 2.0 Chapter 9 Types and Constants - -Gadget drivers -rely on common USB structures and constants -defined in the -<linux/usb/ch9.h> -header file, which is standard in Linux 2.6 kernels. -These are the same types and constants used by host -side drivers (and usbcore). - - -!Iinclude/linux/usb/ch9.h - - -Core Objects and Methods - -These are declared in -<linux/usb/gadget.h>, -and are used by gadget drivers to interact with -USB peripheral controller drivers. - - - - -!Iinclude/linux/usb/gadget.h - - -Optional Utilities - -The core API is sufficient for writing a USB Gadget Driver, -but some optional utilities are provided to simplify common tasks. -These utilities include endpoint autoconfiguration. - - -!Edrivers/usb/gadget/usbstring.c -!Edrivers/usb/gadget/config.c - - - -Composite Device Framework - -The core API is sufficient for writing drivers for composite -USB devices (with more than one function in a given configuration), -and also multi-configuration devices (also more than one function, -but not necessarily sharing a given configuration). -There is however an optional framework which makes it easier to -reuse and combine functions. - - -Devices using this framework provide a struct -usb_composite_driver, which in turn provides one or -more struct usb_configuration instances. -Each such configuration includes at least one -struct usb_function, which packages a user -visible role such as "network link" or "mass storage device". -Management functions may also exist, such as "Device Firmware -Upgrade". - - -!Iinclude/linux/usb/composite.h -!Edrivers/usb/gadget/composite.c - - - -Composite Device Functions - -At this writing, a few of the current gadget drivers have -been converted to this framework. -Near-term plans include converting all of them, except for "gadgetfs". - - -!Edrivers/usb/gadget/function/f_acm.c -!Edrivers/usb/gadget/function/f_ecm.c -!Edrivers/usb/gadget/function/f_subset.c -!Edrivers/usb/gadget/function/f_obex.c -!Edrivers/usb/gadget/function/f_serial.c - - - - - - -Peripheral Controller Drivers - -The first hardware supporting this API was the NetChip 2280 -controller, which supports USB 2.0 high speed and is based on PCI. -This is the net2280 driver module. -The driver supports Linux kernel versions 2.4 and 2.6; -contact NetChip Technologies for development boards and product -information. - - -Other hardware working in the "gadget" framework includes: -Intel's PXA 25x and IXP42x series processors -(pxa2xx_udc), -Toshiba TC86c001 "Goku-S" (goku_udc), -Renesas SH7705/7727 (sh_udc), -MediaQ 11xx (mq11xx_udc), -Hynix HMS30C7202 (h7202_udc), -National 9303/4 (n9604_udc), -Texas Instruments OMAP (omap_udc), -Sharp LH7A40x (lh7a40x_udc), -and more. -Most of those are full speed controllers. - - -At this writing, there are people at work on drivers in -this framework for several other USB device controllers, -with plans to make many of them be widely available. - - - - -A partial USB simulator, -the dummy_hcd driver, is available. -It can act like a net2280, a pxa25x, or an sa11x0 in terms -of available endpoints and device speeds; and it simulates -control, bulk, and to some extent interrupt transfers. -That lets you develop some parts of a gadget driver on a normal PC, -without any special hardware, and perhaps with the assistance -of tools such as GDB running with User Mode Linux. -At least one person has expressed interest in adapting that -approach, hooking it up to a simulator for a microcontroller. -Such simulators can help debug subsystems where the runtime hardware -is unfriendly to software development, or is not yet available. - - -Support for other controllers is expected to be developed -and contributed -over time, as this driver framework evolves. - - - - -Gadget Drivers - -In addition to Gadget Zero -(used primarily for testing and development with drivers -for usb controller hardware), other gadget drivers exist. - - -There's an ethernet gadget -driver, which implements one of the most useful -Communications Device Class (CDC) models. -One of the standards for cable modem interoperability even -specifies the use of this ethernet model as one of two -mandatory options. -Gadgets using this code look to a USB host as if they're -an Ethernet adapter. -It provides access to a network where the gadget's CPU is one host, -which could easily be bridging, routing, or firewalling -access to other networks. -Since some hardware can't fully implement the CDC Ethernet -requirements, this driver also implements a "good parts only" -subset of CDC Ethernet. -(That subset doesn't advertise itself as CDC Ethernet, -to avoid creating problems.) - - -Support for Microsoft's RNDIS -protocol has been contributed by Pengutronix and Auerswald GmbH. -This is like CDC Ethernet, but it runs on more slightly USB hardware -(but less than the CDC subset). -However, its main claim to fame is being able to connect directly to -recent versions of Windows, using drivers that Microsoft bundles -and supports, making it much simpler to network with Windows. - - -There is also support for user mode gadget drivers, -using gadgetfs. -This provides a User Mode API that presents -each endpoint as a single file descriptor. I/O is done using -normal read() and read() calls. -Familiar tools like GDB and pthreads can be used to -develop and debug user mode drivers, so that once a robust -controller driver is available many applications for it -won't require new kernel mode software. -Linux 2.6 Async I/O (AIO) -support is available, so that user mode software -can stream data with only slightly more overhead -than a kernel driver. - - -There's a USB Mass Storage class driver, which provides -a different solution for interoperability with systems such -as MS-Windows and MacOS. -That Mass Storage driver uses a -file or block device as backing store for a drive, -like the loop driver. -The USB host uses the BBB, CB, or CBI versions of the mass -storage class specification, using transparent SCSI commands -to access the data from the backing store. - - -There's a "serial line" driver, useful for TTY style -operation over USB. -The latest version of that driver supports CDC ACM style -operation, like a USB modem, and so on most hardware it can -interoperate easily with MS-Windows. -One interesting use of that driver is in boot firmware (like a BIOS), -which can sometimes use that model with very small systems without -real serial lines. - - -Support for other kinds of gadget is expected to -be developed and contributed -over time, as this driver framework evolves. - - - - -USB On-The-GO (OTG) - -USB OTG support on Linux 2.6 was initially developed -by Texas Instruments for -OMAP 16xx and 17xx -series processors. -Other OTG systems should work in similar ways, but the -hardware level details could be very different. - - -Systems need specialized hardware support to implement OTG, -notably including a special Mini-AB jack -and associated transceiver to support Dual-Role -operation: -they can act either as a host, using the standard -Linux-USB host side driver stack, -or as a peripheral, using this "gadget" framework. -To do that, the system software relies on small additions -to those programming interfaces, -and on a new internal component (here called an "OTG Controller") -affecting which driver stack connects to the OTG port. -In each role, the system can re-use the existing pool of -hardware-neutral drivers, layered on top of the controller -driver interfaces (usb_bus or -usb_gadget). -Such drivers need at most minor changes, and most of the calls -added to support OTG can also benefit non-OTG products. - - - - Gadget drivers test the is_otg - flag, and use it to determine whether or not to include - an OTG descriptor in each of their configurations. - - Gadget drivers may need changes to support the - two new OTG protocols, exposed in new gadget attributes - such as b_hnp_enable flag. - HNP support should be reported through a user interface - (two LEDs could suffice), and is triggered in some cases - when the host suspends the peripheral. - SRP support can be user-initiated just like remote wakeup, - probably by pressing the same button. - - On the host side, USB device drivers need - to be taught to trigger HNP at appropriate moments, using - usb_suspend_device(). - That also conserves battery power, which is useful even - for non-OTG configurations. - - Also on the host side, a driver must support the - OTG "Targeted Peripheral List". That's just a whitelist, - used to reject peripherals not supported with a given - Linux OTG host. - This whitelist is product-specific; - each product must modify otg_whitelist.h - to match its interoperability specification. - - - Non-OTG Linux hosts, like PCs and workstations, - normally have some solution for adding drivers, so that - peripherals that aren't recognized can eventually be supported. - That approach is unreasonable for consumer products that may - never have their firmware upgraded, and where it's usually - unrealistic to expect traditional PC/workstation/server kinds - of support model to work. - For example, it's often impractical to change device firmware - once the product has been distributed, so driver bugs can't - normally be fixed if they're found after shipment. - - - - -Additional changes are needed below those hardware-neutral -usb_bus and usb_gadget -driver interfaces; those aren't discussed here in any detail. -Those affect the hardware-specific code for each USB Host or Peripheral -controller, and how the HCD initializes (since OTG can be active only -on a single port). -They also involve what may be called an OTG Controller -Driver, managing the OTG transceiver and the OTG state -machine logic as well as much of the root hub behavior for the -OTG port. -The OTG controller driver needs to activate and deactivate USB -controllers depending on the relevant device role. -Some related changes were needed inside usbcore, so that it -can identify OTG-capable devices and respond appropriately -to HNP or SRP protocols. - - - - - - diff --git a/Documentation/DocBook/writing_musb_glue_layer.tmpl b/Documentation/DocBook/writing_musb_glue_layer.tmpl deleted file mode 100644 index 837eca77f274..000000000000 --- a/Documentation/DocBook/writing_musb_glue_layer.tmpl +++ /dev/null @@ -1,873 +0,0 @@ - - - - - - Writing an MUSB Glue Layer - - - - Apelete - Seketeli - -
- apelete at seketeli.net -
- - - - - - 2014 - Apelete Seketeli - - - - - This documentation is free software; you can redistribute it - and/or modify it under the terms of the GNU General Public - License as published by the Free Software Foundation; either - version 2 of the License, or (at your option) any later version. - - - - This documentation is distributed in the hope that it will be - useful, but WITHOUT ANY WARRANTY; without even the implied - warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. - See the GNU General Public License for more details. - - - - You should have received a copy of the GNU General Public License - along with this documentation; if not, write to the Free Software - Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA - 02111-1307 USA - - - - For more details see the file COPYING in the Linux kernel source - tree. - - - - - - - - Introduction - - The Linux MUSB subsystem is part of the larger Linux USB - subsystem. It provides support for embedded USB Device Controllers - (UDC) that do not use Universal Host Controller Interface (UHCI) - or Open Host Controller Interface (OHCI). - - - Instead, these embedded UDC rely on the USB On-the-Go (OTG) - specification which they implement at least partially. The silicon - reference design used in most cases is the Multipoint USB - Highspeed Dual-Role Controller (MUSB HDRC) found in the Mentor - Graphics Inventra™ design. - - - As a self-taught exercise I have written an MUSB glue layer for - the Ingenic JZ4740 SoC, modelled after the many MUSB glue layers - in the kernel source tree. This layer can be found at - drivers/usb/musb/jz4740.c. In this documentation I will walk - through the basics of the jz4740.c glue layer, explaining the - different pieces and what needs to be done in order to write your - own device glue layer. - - - - - Linux MUSB Basics - - To get started on the topic, please read USB On-the-Go Basics (see - Resources) which provides an introduction of USB OTG operation at - the hardware level. A couple of wiki pages by Texas Instruments - and Analog Devices also provide an overview of the Linux kernel - MUSB configuration, albeit focused on some specific devices - provided by these companies. Finally, getting acquainted with the - USB specification at USB home page may come in handy, with - practical instance provided through the Writing USB Device Drivers - documentation (again, see Resources). - - - Linux USB stack is a layered architecture in which the MUSB - controller hardware sits at the lowest. The MUSB controller driver - abstract the MUSB controller hardware to the Linux USB stack. - - - ------------------------ - | | <------- drivers/usb/gadget - | Linux USB Core Stack | <------- drivers/usb/host - | | <------- drivers/usb/core - ------------------------ - ⬍ - -------------------------- - | | <------ drivers/usb/musb/musb_gadget.c - | MUSB Controller driver | <------ drivers/usb/musb/musb_host.c - | | <------ drivers/usb/musb/musb_core.c - -------------------------- - ⬍ - --------------------------------- - | MUSB Platform Specific Driver | - | | <-- drivers/usb/musb/jz4740.c - | aka "Glue Layer" | - --------------------------------- - ⬍ - --------------------------------- - | MUSB Controller Hardware | - --------------------------------- - - - As outlined above, the glue layer is actually the platform - specific code sitting in between the controller driver and the - controller hardware. - - - Just like a Linux USB driver needs to register itself with the - Linux USB subsystem, the MUSB glue layer needs first to register - itself with the MUSB controller driver. This will allow the - controller driver to know about which device the glue layer - supports and which functions to call when a supported device is - detected or released; remember we are talking about an embedded - controller chip here, so no insertion or removal at run-time. - - - All of this information is passed to the MUSB controller driver - through a platform_driver structure defined in the glue layer as: - - -static struct platform_driver jz4740_driver = { - .probe = jz4740_probe, - .remove = jz4740_remove, - .driver = { - .name = "musb-jz4740", - }, -}; - - - The probe and remove function pointers are called when a matching - device is detected and, respectively, released. The name string - describes the device supported by this glue layer. In the current - case it matches a platform_device structure declared in - arch/mips/jz4740/platform.c. Note that we are not using device - tree bindings here. - - - In order to register itself to the controller driver, the glue - layer goes through a few steps, basically allocating the - controller hardware resources and initialising a couple of - circuits. To do so, it needs to keep track of the information used - throughout these steps. This is done by defining a private - jz4740_glue structure: - - -struct jz4740_glue { - struct device *dev; - struct platform_device *musb; - struct clk *clk; -}; - - - The dev and musb members are both device structure variables. The - first one holds generic information about the device, since it's - the basic device structure, and the latter holds information more - closely related to the subsystem the device is registered to. The - clk variable keeps information related to the device clock - operation. - - - Let's go through the steps of the probe function that leads the - glue layer to register itself to the controller driver. - - - N.B.: For the sake of readability each function will be split in - logical parts, each part being shown as if it was independent from - the others. - - -static int jz4740_probe(struct platform_device *pdev) -{ - struct platform_device *musb; - struct jz4740_glue *glue; - struct clk *clk; - int ret; - - glue = devm_kzalloc(&pdev->dev, sizeof(*glue), GFP_KERNEL); - if (!glue) - return -ENOMEM; - - musb = platform_device_alloc("musb-hdrc", PLATFORM_DEVID_AUTO); - if (!musb) { - dev_err(&pdev->dev, "failed to allocate musb device\n"); - return -ENOMEM; - } - - clk = devm_clk_get(&pdev->dev, "udc"); - if (IS_ERR(clk)) { - dev_err(&pdev->dev, "failed to get clock\n"); - ret = PTR_ERR(clk); - goto err_platform_device_put; - } - - ret = clk_prepare_enable(clk); - if (ret) { - dev_err(&pdev->dev, "failed to enable clock\n"); - goto err_platform_device_put; - } - - musb->dev.parent = &pdev->dev; - - glue->dev = &pdev->dev; - glue->musb = musb; - glue->clk = clk; - - return 0; - -err_platform_device_put: - platform_device_put(musb); - return ret; -} - - - The first few lines of the probe function allocate and assign the - glue, musb and clk variables. The GFP_KERNEL flag (line 8) allows - the allocation process to sleep and wait for memory, thus being - usable in a blocking situation. The PLATFORM_DEVID_AUTO flag (line - 12) allows automatic allocation and management of device IDs in - order to avoid device namespace collisions with explicit IDs. With - devm_clk_get() (line 18) the glue layer allocates the clock -- the - devm_ prefix indicates that clk_get() is - managed: it automatically frees the allocated clock resource data - when the device is released -- and enable it. - - - Then comes the registration steps: - - -static int jz4740_probe(struct platform_device *pdev) -{ - struct musb_hdrc_platform_data *pdata = &jz4740_musb_platform_data; - - pdata->platform_ops = &jz4740_musb_ops; - - platform_set_drvdata(pdev, glue); - - ret = platform_device_add_resources(musb, pdev->resource, - pdev->num_resources); - if (ret) { - dev_err(&pdev->dev, "failed to add resources\n"); - goto err_clk_disable; - } - - ret = platform_device_add_data(musb, pdata, sizeof(*pdata)); - if (ret) { - dev_err(&pdev->dev, "failed to add platform_data\n"); - goto err_clk_disable; - } - - return 0; - -err_clk_disable: - clk_disable_unprepare(clk); -err_platform_device_put: - platform_device_put(musb); - return ret; -} - - - The first step is to pass the device data privately held by the - glue layer on to the controller driver through - platform_set_drvdata() (line 7). Next is passing on the device - resources information, also privately held at that point, through - platform_device_add_resources() (line 9). - - - Finally comes passing on the platform specific data to the - controller driver (line 16). Platform data will be discussed in - Chapter 4, but here - we are looking at the platform_ops function pointer (line 5) in - musb_hdrc_platform_data structure (line 3). This function - pointer allows the MUSB controller driver to know which function - to call for device operation: - - -static const struct musb_platform_ops jz4740_musb_ops = { - .init = jz4740_musb_init, - .exit = jz4740_musb_exit, -}; - - - Here we have the minimal case where only init and exit functions - are called by the controller driver when needed. Fact is the - JZ4740 MUSB controller is a basic controller, lacking some - features found in other controllers, otherwise we may also have - pointers to a few other functions like a power management function - or a function to switch between OTG and non-OTG modes, for - instance. - - - At that point of the registration process, the controller driver - actually calls the init function: - - -static int jz4740_musb_init(struct musb *musb) -{ - musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2); - if (!musb->xceiv) { - pr_err("HS UDC: no transceiver configured\n"); - return -ENODEV; - } - - /* Silicon does not implement ConfigData register. - * Set dyn_fifo to avoid reading EP config from hardware. - */ - musb->dyn_fifo = true; - - musb->isr = jz4740_musb_interrupt; - - return 0; -} - - - The goal of jz4740_musb_init() is to get hold of the transceiver - driver data of the MUSB controller hardware and pass it on to the - MUSB controller driver, as usual. The transceiver is the circuitry - inside the controller hardware responsible for sending/receiving - the USB data. Since it is an implementation of the physical layer - of the OSI model, the transceiver is also referred to as PHY. - - - Getting hold of the MUSB PHY driver data is done with - usb_get_phy() which returns a pointer to the structure - containing the driver instance data. The next couple of - instructions (line 12 and 14) are used as a quirk and to setup - IRQ handling respectively. Quirks and IRQ handling will be - discussed later in Chapter - 5 and Chapter 3. - - -static int jz4740_musb_exit(struct musb *musb) -{ - usb_put_phy(musb->xceiv); - - return 0; -} - - - Acting as the counterpart of init, the exit function releases the - MUSB PHY driver when the controller hardware itself is about to be - released. - - - Again, note that init and exit are fairly simple in this case due - to the basic set of features of the JZ4740 controller hardware. - When writing an musb glue layer for a more complex controller - hardware, you might need to take care of more processing in those - two functions. - - - Returning from the init function, the MUSB controller driver jumps - back into the probe function: - - -static int jz4740_probe(struct platform_device *pdev) -{ - ret = platform_device_add(musb); - if (ret) { - dev_err(&pdev->dev, "failed to register musb device\n"); - goto err_clk_disable; - } - - return 0; - -err_clk_disable: - clk_disable_unprepare(clk); -err_platform_device_put: - platform_device_put(musb); - return ret; -} - - - This is the last part of the device registration process where the - glue layer adds the controller hardware device to Linux kernel - device hierarchy: at this stage, all known information about the - device is passed on to the Linux USB core stack. - - -static int jz4740_remove(struct platform_device *pdev) -{ - struct jz4740_glue *glue = platform_get_drvdata(pdev); - - platform_device_unregister(glue->musb); - clk_disable_unprepare(glue->clk); - - return 0; -} - - - Acting as the counterpart of probe, the remove function unregister - the MUSB controller hardware (line 5) and disable the clock (line - 6), allowing it to be gated. - - - - - Handling IRQs - - Additionally to the MUSB controller hardware basic setup and - registration, the glue layer is also responsible for handling the - IRQs: - - -static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci) -{ - unsigned long flags; - irqreturn_t retval = IRQ_NONE; - struct musb *musb = __hci; - - spin_lock_irqsave(&musb->lock, flags); - - musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB); - musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX); - musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX); - - /* - * The controller is gadget only, the state of the host mode IRQ bits is - * undefined. Mask them to make sure that the musb driver core will - * never see them set - */ - musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME | - MUSB_INTR_RESET | MUSB_INTR_SOF; - - if (musb->int_usb || musb->int_tx || musb->int_rx) - retval = musb_interrupt(musb); - - spin_unlock_irqrestore(&musb->lock, flags); - - return retval; -} - - - Here the glue layer mostly has to read the relevant hardware - registers and pass their values on to the controller driver which - will handle the actual event that triggered the IRQ. - - - The interrupt handler critical section is protected by the - spin_lock_irqsave() and counterpart spin_unlock_irqrestore() - functions (line 7 and 24 respectively), which prevent the - interrupt handler code to be run by two different threads at the - same time. - - - Then the relevant interrupt registers are read (line 9 to 11): - - - - - MUSB_INTRUSB: indicates which USB interrupts are currently - active, - - - - - MUSB_INTRTX: indicates which of the interrupts for TX - endpoints are currently active, - - - - - MUSB_INTRRX: indicates which of the interrupts for TX - endpoints are currently active. - - - - - Note that musb_readb() is used to read 8-bit registers at most, - while musb_readw() allows us to read at most 16-bit registers. - There are other functions that can be used depending on the size - of your device registers. See musb_io.h for more information. - - - Instruction on line 18 is another quirk specific to the JZ4740 - USB device controller, which will be discussed later in Chapter 5. - - - The glue layer still needs to register the IRQ handler though. - Remember the instruction on line 14 of the init function: - - -static int jz4740_musb_init(struct musb *musb) -{ - musb->isr = jz4740_musb_interrupt; - - return 0; -} - - - This instruction sets a pointer to the glue layer IRQ handler - function, in order for the controller hardware to call the handler - back when an IRQ comes from the controller hardware. The interrupt - handler is now implemented and registered. - - - - - Device Platform Data - - In order to write an MUSB glue layer, you need to have some data - describing the hardware capabilities of your controller hardware, - which is called the platform data. - - - Platform data is specific to your hardware, though it may cover a - broad range of devices, and is generally found somewhere in the - arch/ directory, depending on your device architecture. - - - For instance, platform data for the JZ4740 SoC is found in - arch/mips/jz4740/platform.c. In the platform.c file each device of - the JZ4740 SoC is described through a set of structures. - - - Here is the part of arch/mips/jz4740/platform.c that covers the - USB Device Controller (UDC): - - -/* USB Device Controller */ -struct platform_device jz4740_udc_xceiv_device = { - .name = "usb_phy_gen_xceiv", - .id = 0, -}; - -static struct resource jz4740_udc_resources[] = { - [0] = { - .start = JZ4740_UDC_BASE_ADDR, - .end = JZ4740_UDC_BASE_ADDR + 0x10000 - 1, - .flags = IORESOURCE_MEM, - }, - [1] = { - .start = JZ4740_IRQ_UDC, - .end = JZ4740_IRQ_UDC, - .flags = IORESOURCE_IRQ, - .name = "mc", - }, -}; - -struct platform_device jz4740_udc_device = { - .name = "musb-jz4740", - .id = -1, - .dev = { - .dma_mask = &jz4740_udc_device.dev.coherent_dma_mask, - .coherent_dma_mask = DMA_BIT_MASK(32), - }, - .num_resources = ARRAY_SIZE(jz4740_udc_resources), - .resource = jz4740_udc_resources, -}; - - - The jz4740_udc_xceiv_device platform device structure (line 2) - describes the UDC transceiver with a name and id number. - - - At the time of this writing, note that - "usb_phy_gen_xceiv" is the specific name to be used for - all transceivers that are either built-in with reference USB IP or - autonomous and doesn't require any PHY programming. You will need - to set CONFIG_NOP_USB_XCEIV=y in the kernel configuration to make - use of the corresponding transceiver driver. The id field could be - set to -1 (equivalent to PLATFORM_DEVID_NONE), -2 (equivalent to - PLATFORM_DEVID_AUTO) or start with 0 for the first device of this - kind if we want a specific id number. - - - The jz4740_udc_resources resource structure (line 7) defines the - UDC registers base addresses. - - - The first array (line 9 to 11) defines the UDC registers base - memory addresses: start points to the first register memory - address, end points to the last register memory address and the - flags member defines the type of resource we are dealing with. So - IORESOURCE_MEM is used to define the registers memory addresses. - The second array (line 14 to 17) defines the UDC IRQ registers - addresses. Since there is only one IRQ register available for the - JZ4740 UDC, start and end point at the same address. The - IORESOURCE_IRQ flag tells that we are dealing with IRQ resources, - and the name "mc" is in fact hard-coded in the MUSB core - in order for the controller driver to retrieve this IRQ resource - by querying it by its name. - - - Finally, the jz4740_udc_device platform device structure (line 21) - describes the UDC itself. - - - The "musb-jz4740" name (line 22) defines the MUSB - driver that is used for this device; remember this is in fact - the name that we used in the jz4740_driver platform driver - structure in Chapter - 2. The id field (line 23) is set to -1 (equivalent to - PLATFORM_DEVID_NONE) since we do not need an id for the device: - the MUSB controller driver was already set to allocate an - automatic id in Chapter - 2. In the dev field we care for DMA related information - here. The dma_mask field (line 25) defines the width of the DMA - mask that is going to be used, and coherent_dma_mask (line 26) - has the same purpose but for the alloc_coherent DMA mappings: in - both cases we are using a 32 bits mask. Then the resource field - (line 29) is simply a pointer to the resource structure defined - before, while the num_resources field (line 28) keeps track of - the number of arrays defined in the resource structure (in this - case there were two resource arrays defined before). - - - With this quick overview of the UDC platform data at the arch/ - level now done, let's get back to the MUSB glue layer specific - platform data in drivers/usb/musb/jz4740.c: - - -static struct musb_hdrc_config jz4740_musb_config = { - /* Silicon does not implement USB OTG. */ - .multipoint = 0, - /* Max EPs scanned, driver will decide which EP can be used. */ - .num_eps = 4, - /* RAMbits needed to configure EPs from table */ - .ram_bits = 9, - .fifo_cfg = jz4740_musb_fifo_cfg, - .fifo_cfg_size = ARRAY_SIZE(jz4740_musb_fifo_cfg), -}; - -static struct musb_hdrc_platform_data jz4740_musb_platform_data = { - .mode = MUSB_PERIPHERAL, - .config = &jz4740_musb_config, -}; - - - First the glue layer configures some aspects of the controller - driver operation related to the controller hardware specifics. - This is done through the jz4740_musb_config musb_hdrc_config - structure. - - - Defining the OTG capability of the controller hardware, the - multipoint member (line 3) is set to 0 (equivalent to false) - since the JZ4740 UDC is not OTG compatible. Then num_eps (line - 5) defines the number of USB endpoints of the controller - hardware, including endpoint 0: here we have 3 endpoints + - endpoint 0. Next is ram_bits (line 7) which is the width of the - RAM address bus for the MUSB controller hardware. This - information is needed when the controller driver cannot - automatically configure endpoints by reading the relevant - controller hardware registers. This issue will be discussed when - we get to device quirks in Chapter - 5. Last two fields (line 8 and 9) are also about device - quirks: fifo_cfg points to the USB endpoints configuration table - and fifo_cfg_size keeps track of the size of the number of - entries in that configuration table. More on that later in Chapter 5. - - - Then this configuration is embedded inside - jz4740_musb_platform_data musb_hdrc_platform_data structure (line - 11): config is a pointer to the configuration structure itself, - and mode tells the controller driver if the controller hardware - may be used as MUSB_HOST only, MUSB_PERIPHERAL only or MUSB_OTG - which is a dual mode. - - - Remember that jz4740_musb_platform_data is then used to convey - platform data information as we have seen in the probe function - in Chapter 2 - - - - - Device Quirks - - Completing the platform data specific to your device, you may also - need to write some code in the glue layer to work around some - device specific limitations. These quirks may be due to some - hardware bugs, or simply be the result of an incomplete - implementation of the USB On-the-Go specification. - - - The JZ4740 UDC exhibits such quirks, some of which we will discuss - here for the sake of insight even though these might not be found - in the controller hardware you are working on. - - - Let's get back to the init function first: - - -static int jz4740_musb_init(struct musb *musb) -{ - musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2); - if (!musb->xceiv) { - pr_err("HS UDC: no transceiver configured\n"); - return -ENODEV; - } - - /* Silicon does not implement ConfigData register. - * Set dyn_fifo to avoid reading EP config from hardware. - */ - musb->dyn_fifo = true; - - musb->isr = jz4740_musb_interrupt; - - return 0; -} - - - Instruction on line 12 helps the MUSB controller driver to work - around the fact that the controller hardware is missing registers - that are used for USB endpoints configuration. - - - Without these registers, the controller driver is unable to read - the endpoints configuration from the hardware, so we use line 12 - instruction to bypass reading the configuration from silicon, and - rely on a hard-coded table that describes the endpoints - configuration instead: - - -static struct musb_fifo_cfg jz4740_musb_fifo_cfg[] = { -{ .hw_ep_num = 1, .style = FIFO_TX, .maxpacket = 512, }, -{ .hw_ep_num = 1, .style = FIFO_RX, .maxpacket = 512, }, -{ .hw_ep_num = 2, .style = FIFO_TX, .maxpacket = 64, }, -}; - - - Looking at the configuration table above, we see that each - endpoints is described by three fields: hw_ep_num is the endpoint - number, style is its direction (either FIFO_TX for the controller - driver to send packets in the controller hardware, or FIFO_RX to - receive packets from hardware), and maxpacket defines the maximum - size of each data packet that can be transmitted over that - endpoint. Reading from the table, the controller driver knows that - endpoint 1 can be used to send and receive USB data packets of 512 - bytes at once (this is in fact a bulk in/out endpoint), and - endpoint 2 can be used to send data packets of 64 bytes at once - (this is in fact an interrupt endpoint). - - - Note that there is no information about endpoint 0 here: that one - is implemented by default in every silicon design, with a - predefined configuration according to the USB specification. For - more examples of endpoint configuration tables, see musb_core.c. - - - Let's now get back to the interrupt handler function: - - -static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci) -{ - unsigned long flags; - irqreturn_t retval = IRQ_NONE; - struct musb *musb = __hci; - - spin_lock_irqsave(&musb->lock, flags); - - musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB); - musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX); - musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX); - - /* - * The controller is gadget only, the state of the host mode IRQ bits is - * undefined. Mask them to make sure that the musb driver core will - * never see them set - */ - musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME | - MUSB_INTR_RESET | MUSB_INTR_SOF; - - if (musb->int_usb || musb->int_tx || musb->int_rx) - retval = musb_interrupt(musb); - - spin_unlock_irqrestore(&musb->lock, flags); - - return retval; -} - - - Instruction on line 18 above is a way for the controller driver to - work around the fact that some interrupt bits used for USB host - mode operation are missing in the MUSB_INTRUSB register, thus left - in an undefined hardware state, since this MUSB controller - hardware is used in peripheral mode only. As a consequence, the - glue layer masks these missing bits out to avoid parasite - interrupts by doing a logical AND operation between the value read - from MUSB_INTRUSB and the bits that are actually implemented in - the register. - - - These are only a couple of the quirks found in the JZ4740 USB - device controller. Some others were directly addressed in the MUSB - core since the fixes were generic enough to provide a better - handling of the issues for others controller hardware eventually. - - - - - Conclusion - - Writing a Linux MUSB glue layer should be a more accessible task, - as this documentation tries to show the ins and outs of this - exercise. - - - The JZ4740 USB device controller being fairly simple, I hope its - glue layer serves as a good example for the curious mind. Used - with the current MUSB glue layers, this documentation should - provide enough guidance to get started; should anything gets out - of hand, the linux-usb mailing list archive is another helpful - resource to browse through. - - - - - Acknowledgements - - Many thanks to Lars-Peter Clausen and Maarten ter Huurne for - answering my questions while I was writing the JZ4740 glue layer - and for helping me out getting the code in good shape. - - - I would also like to thank the Qi-Hardware community at large for - its cheerful guidance and support. - - - - - Resources - - USB Home Page: - http://www.usb.org - - - linux-usb Mailing List Archives: - http://marc.info/?l=linux-usb - - - USB On-the-Go Basics: - http://www.maximintegrated.com/app-notes/index.mvp/id/1822 - - - Writing USB Device Drivers: - https://www.kernel.org/doc/htmldocs/writing_usb_driver/index.html - - - Texas Instruments USB Configuration Wiki Page: - http://processors.wiki.ti.com/index.php/Usbgeneralpage - - - Analog Devices Blackfin MUSB Configuration: - http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:musb - - - - diff --git a/Documentation/DocBook/writing_usb_driver.tmpl b/Documentation/DocBook/writing_usb_driver.tmpl deleted file mode 100644 index 3210dcf741c9..000000000000 --- a/Documentation/DocBook/writing_usb_driver.tmpl +++ /dev/null @@ -1,412 +0,0 @@ - - - - - - Writing USB Device Drivers - - - - Greg - Kroah-Hartman - -
- greg@kroah.com -
- - - - - - 2001-2002 - Greg Kroah-Hartman - - - - - This documentation is free software; you can redistribute - it and/or modify it under the terms of the GNU General Public - License as published by the Free Software Foundation; either - version 2 of the License, or (at your option) any later - version. - - - - This program is distributed in the hope that it will be - useful, but WITHOUT ANY WARRANTY; without even the implied - warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. - See the GNU General Public License for more details. - - - - You should have received a copy of the GNU General Public - License along with this program; if not, write to the Free - Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, - MA 02111-1307 USA - - - - For more details see the file COPYING in the source - distribution of Linux. - - - - This documentation is based on an article published in - Linux Journal Magazine, October 2001, Issue 90. - - - - - - - - Introduction - - The Linux USB subsystem has grown from supporting only two different - types of devices in the 2.2.7 kernel (mice and keyboards), to over 20 - different types of devices in the 2.4 kernel. Linux currently supports - almost all USB class devices (standard types of devices like keyboards, - mice, modems, printers and speakers) and an ever-growing number of - vendor-specific devices (such as USB to serial converters, digital - cameras, Ethernet devices and MP3 players). For a full list of the - different USB devices currently supported, see Resources. - - - The remaining kinds of USB devices that do not have support on Linux are - almost all vendor-specific devices. Each vendor decides to implement a - custom protocol to talk to their device, so a custom driver usually needs - to be created. Some vendors are open with their USB protocols and help - with the creation of Linux drivers, while others do not publish them, and - developers are forced to reverse-engineer. See Resources for some links - to handy reverse-engineering tools. - - - Because each different protocol causes a new driver to be created, I have - written a generic USB driver skeleton, modelled after the pci-skeleton.c - file in the kernel source tree upon which many PCI network drivers have - been based. This USB skeleton can be found at drivers/usb/usb-skeleton.c - in the kernel source tree. In this article I will walk through the basics - of the skeleton driver, explaining the different pieces and what needs to - be done to customize it to your specific device. - - - - - Linux USB Basics - - If you are going to write a Linux USB driver, please become familiar with - the USB protocol specification. It can be found, along with many other - useful documents, at the USB home page (see Resources). An excellent - introduction to the Linux USB subsystem can be found at the USB Working - Devices List (see Resources). It explains how the Linux USB subsystem is - structured and introduces the reader to the concept of USB urbs - (USB Request Blocks), which are essential to USB drivers. - - - The first thing a Linux USB driver needs to do is register itself with - the Linux USB subsystem, giving it some information about which devices - the driver supports and which functions to call when a device supported - by the driver is inserted or removed from the system. All of this - information is passed to the USB subsystem in the usb_driver structure. - The skeleton driver declares a usb_driver as: - - -static struct usb_driver skel_driver = { - .name = "skeleton", - .probe = skel_probe, - .disconnect = skel_disconnect, - .fops = &skel_fops, - .minor = USB_SKEL_MINOR_BASE, - .id_table = skel_table, -}; - - - The variable name is a string that describes the driver. It is used in - informational messages printed to the system log. The probe and - disconnect function pointers are called when a device that matches the - information provided in the id_table variable is either seen or removed. - - - The fops and minor variables are optional. Most USB drivers hook into - another kernel subsystem, such as the SCSI, network or TTY subsystem. - These types of drivers register themselves with the other kernel - subsystem, and any user-space interactions are provided through that - interface. But for drivers that do not have a matching kernel subsystem, - such as MP3 players or scanners, a method of interacting with user space - is needed. The USB subsystem provides a way to register a minor device - number and a set of file_operations function pointers that enable this - user-space interaction. The skeleton driver needs this kind of interface, - so it provides a minor starting number and a pointer to its - file_operations functions. - - - The USB driver is then registered with a call to usb_register, usually in - the driver's init function, as shown here: - - -static int __init usb_skel_init(void) -{ - int result; - - /* register this driver with the USB subsystem */ - result = usb_register(&skel_driver); - if (result < 0) { - err("usb_register failed for the "__FILE__ "driver." - "Error number %d", result); - return -1; - } - - return 0; -} -module_init(usb_skel_init); - - - When the driver is unloaded from the system, it needs to deregister - itself with the USB subsystem. This is done with the usb_deregister - function: - - -static void __exit usb_skel_exit(void) -{ - /* deregister this driver with the USB subsystem */ - usb_deregister(&skel_driver); -} -module_exit(usb_skel_exit); - - - To enable the linux-hotplug system to load the driver automatically when - the device is plugged in, you need to create a MODULE_DEVICE_TABLE. The - following code tells the hotplug scripts that this module supports a - single device with a specific vendor and product ID: - - -/* table of devices that work with this driver */ -static struct usb_device_id skel_table [] = { - { USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) }, - { } /* Terminating entry */ -}; -MODULE_DEVICE_TABLE (usb, skel_table); - - - There are other macros that can be used in describing a usb_device_id for - drivers that support a whole class of USB drivers. See usb.h for more - information on this. - - - - - Device operation - - When a device is plugged into the USB bus that matches the device ID - pattern that your driver registered with the USB core, the probe function - is called. The usb_device structure, interface number and the interface ID - are passed to the function: - - -static int skel_probe(struct usb_interface *interface, - const struct usb_device_id *id) - - - The driver now needs to verify that this device is actually one that it - can accept. If so, it returns 0. - If not, or if any error occurs during initialization, an errorcode - (such as -ENOMEM or -ENODEV) - is returned from the probe function. - - - In the skeleton driver, we determine what end points are marked as bulk-in - and bulk-out. We create buffers to hold the data that will be sent and - received from the device, and a USB urb to write data to the device is - initialized. - - - Conversely, when the device is removed from the USB bus, the disconnect - function is called with the device pointer. The driver needs to clean any - private data that has been allocated at this time and to shut down any - pending urbs that are in the USB system. - - - Now that the device is plugged into the system and the driver is bound to - the device, any of the functions in the file_operations structure that - were passed to the USB subsystem will be called from a user program trying - to talk to the device. The first function called will be open, as the - program tries to open the device for I/O. We increment our private usage - count and save a pointer to our internal structure in the file - structure. This is done so that future calls to file operations will - enable the driver to determine which device the user is addressing. All - of this is done with the following code: - - -/* increment our usage count for the module */ -++skel->open_count; - -/* save our object in the file's private structure */ -file->private_data = dev; - - - After the open function is called, the read and write functions are called - to receive and send data to the device. In the skel_write function, we - receive a pointer to some data that the user wants to send to the device - and the size of the data. The function determines how much data it can - send to the device based on the size of the write urb it has created (this - size depends on the size of the bulk out end point that the device has). - Then it copies the data from user space to kernel space, points the urb to - the data and submits the urb to the USB subsystem. This can be seen in - the following code: - - -/* we can only write as much as 1 urb will hold */ -bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count; - -/* copy the data from user space into our urb */ -copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written); - -/* set up our urb */ -usb_fill_bulk_urb(skel->write_urb, - skel->dev, - usb_sndbulkpipe(skel->dev, skel->bulk_out_endpointAddr), - skel->write_urb->transfer_buffer, - bytes_written, - skel_write_bulk_callback, - skel); - -/* send the data out the bulk port */ -result = usb_submit_urb(skel->write_urb); -if (result) { - err("Failed submitting write urb, error %d", result); -} - - - When the write urb is filled up with the proper information using the - usb_fill_bulk_urb function, we point the urb's completion callback to call our - own skel_write_bulk_callback function. This function is called when the - urb is finished by the USB subsystem. The callback function is called in - interrupt context, so caution must be taken not to do very much processing - at that time. Our implementation of skel_write_bulk_callback merely - reports if the urb was completed successfully or not and then returns. - - - The read function works a bit differently from the write function in that - we do not use an urb to transfer data from the device to the driver. - Instead we call the usb_bulk_msg function, which can be used to send or - receive data from a device without having to create urbs and handle - urb completion callback functions. We call the usb_bulk_msg function, - giving it a buffer into which to place any data received from the device - and a timeout value. If the timeout period expires without receiving any - data from the device, the function will fail and return an error message. - This can be shown with the following code: - - -/* do an immediate bulk read to get data from the device */ -retval = usb_bulk_msg (skel->dev, - usb_rcvbulkpipe (skel->dev, - skel->bulk_in_endpointAddr), - skel->bulk_in_buffer, - skel->bulk_in_size, - &count, HZ*10); -/* if the read was successful, copy the data to user space */ -if (!retval) { - if (copy_to_user (buffer, skel->bulk_in_buffer, count)) - retval = -EFAULT; - else - retval = count; -} - - - The usb_bulk_msg function can be very useful for doing single reads or - writes to a device; however, if you need to read or write constantly to a - device, it is recommended to set up your own urbs and submit them to the - USB subsystem. - - - When the user program releases the file handle that it has been using to - talk to the device, the release function in the driver is called. In this - function we decrement our private usage count and wait for possible - pending writes: - - -/* decrement our usage count for the device */ ---skel->open_count; - - - One of the more difficult problems that USB drivers must be able to handle - smoothly is the fact that the USB device may be removed from the system at - any point in time, even if a program is currently talking to it. It needs - to be able to shut down any current reads and writes and notify the - user-space programs that the device is no longer there. The following - code (function skel_delete) - is an example of how to do this: - -static inline void skel_delete (struct usb_skel *dev) -{ - kfree (dev->bulk_in_buffer); - if (dev->bulk_out_buffer != NULL) - usb_free_coherent (dev->udev, dev->bulk_out_size, - dev->bulk_out_buffer, - dev->write_urb->transfer_dma); - usb_free_urb (dev->write_urb); - kfree (dev); -} - - - If a program currently has an open handle to the device, we reset the flag - device_present. For - every read, write, release and other functions that expect a device to be - present, the driver first checks this flag to see if the device is - still present. If not, it releases that the device has disappeared, and a - -ENODEV error is returned to the user-space program. When the release - function is eventually called, it determines if there is no device - and if not, it does the cleanup that the skel_disconnect - function normally does if there are no open files on the device (see - Listing 5). - - - - - Isochronous Data - - This usb-skeleton driver does not have any examples of interrupt or - isochronous data being sent to or from the device. Interrupt data is sent - almost exactly as bulk data is, with a few minor exceptions. Isochronous - data works differently with continuous streams of data being sent to or - from the device. The audio and video camera drivers are very good examples - of drivers that handle isochronous data and will be useful if you also - need to do this. - - - - - Conclusion - - Writing Linux USB device drivers is not a difficult task as the - usb-skeleton driver shows. This driver, combined with the other current - USB drivers, should provide enough examples to help a beginning author - create a working driver in a minimal amount of time. The linux-usb-devel - mailing list archives also contain a lot of helpful information. - - - - - Resources - - The Linux USB Project: http://www.linux-usb.org/ - - - Linux Hotplug Project: http://linux-hotplug.sourceforge.net/ - - - Linux USB Working Devices List: http://www.qbik.ch/usb/devices/ - - - linux-usb-devel Mailing List Archives: http://marc.theaimsgroup.com/?l=linux-usb-devel - - - Programming Guide for Linux USB Device Drivers: http://usb.cs.tum.edu/usbdoc - - - USB Home Page: http://www.usb.org - - - - diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst index 585982e36b3d..8058a87c1c74 100644 --- a/Documentation/driver-api/index.rst +++ b/Documentation/driver-api/index.rst @@ -26,7 +26,7 @@ available subsections can be seen below. regulator iio/index input - usb + usb/index pci spi i2c diff --git a/Documentation/driver-api/usb/gadget.rst b/Documentation/driver-api/usb/gadget.rst new file mode 100644 index 000000000000..52b299b1ca6d --- /dev/null +++ b/Documentation/driver-api/usb/gadget.rst @@ -0,0 +1,533 @@ +======================== +USB Gadget API for Linux +======================== + +:Author: David Brownell +:Date: 20 August 2004 + +Introduction +============ + +This document presents a Linux-USB "Gadget" kernel mode API, for use +within peripherals and other USB devices that embed Linux. It provides +an overview of the API structure, and shows how that fits into a system +development project. This is the first such API released on Linux to +address a number of important problems, including: + +- Supports USB 2.0, for high speed devices which can stream data at + several dozen megabytes per second. + +- Handles devices with dozens of endpoints just as well as ones with + just two fixed-function ones. Gadget drivers can be written so + they're easy to port to new hardware. + +- Flexible enough to expose more complex USB device capabilities such + as multiple configurations, multiple interfaces, composite devices, + and alternate interface settings. + +- USB "On-The-Go" (OTG) support, in conjunction with updates to the + Linux-USB host side. + +- Sharing data structures and API models with the Linux-USB host side + API. This helps the OTG support, and looks forward to more-symmetric + frameworks (where the same I/O model is used by both host and device + side drivers). + +- Minimalist, so it's easier to support new device controller hardware. + I/O processing doesn't imply large demands for memory or CPU + resources. + +Most Linux developers will not be able to use this API, since they have +USB "host" hardware in a PC, workstation, or server. Linux users with +embedded systems are more likely to have USB peripheral hardware. To +distinguish drivers running inside such hardware from the more familiar +Linux "USB device drivers", which are host side proxies for the real USB +devices, a different term is used: the drivers inside the peripherals +are "USB gadget drivers". In USB protocol interactions, the device +driver is the master (or "client driver") and the gadget driver is the +slave (or "function driver"). + +The gadget API resembles the host side Linux-USB API in that both use +queues of request objects to package I/O buffers, and those requests may +be submitted or canceled. They share common definitions for the standard +USB *Chapter 9* messages, structures, and constants. Also, both APIs +bind and unbind drivers to devices. The APIs differ in detail, since the +host side's current URB framework exposes a number of implementation +details and assumptions that are inappropriate for a gadget API. While +the model for control transfers and configuration management is +necessarily different (one side is a hardware-neutral master, the other +is a hardware-aware slave), the endpoint I/0 API used here should also +be usable for an overhead-reduced host side API. + +Structure of Gadget Drivers +=========================== + +A system running inside a USB peripheral normally has at least three +layers inside the kernel to handle USB protocol processing, and may have +additional layers in user space code. The "gadget" API is used by the +middle layer to interact with the lowest level (which directly handles +hardware). + +In Linux, from the bottom up, these layers are: + +*USB Controller Driver* + This is the lowest software level. It is the only layer that talks + to hardware, through registers, fifos, dma, irqs, and the like. The + ```` API abstracts the peripheral controller + endpoint hardware. That hardware is exposed through endpoint + objects, which accept streams of IN/OUT buffers, and through + callbacks that interact with gadget drivers. Since normal USB + devices only have one upstream port, they only have one of these + drivers. The controller driver can support any number of different + gadget drivers, but only one of them can be used at a time. + + Examples of such controller hardware include the PCI-based NetChip + 2280 USB 2.0 high speed controller, the SA-11x0 or PXA-25x UDC + (found within many PDAs), and a variety of other products. + +*Gadget Driver* + The lower boundary of this driver implements hardware-neutral USB + functions, using calls to the controller driver. Because such + hardware varies widely in capabilities and restrictions, and is used + in embedded environments where space is at a premium, the gadget + driver is often configured at compile time to work with endpoints + supported by one particular controller. Gadget drivers may be + portable to several different controllers, using conditional + compilation. (Recent kernels substantially simplify the work + involved in supporting new hardware, by *autoconfiguring* endpoints + automatically for many bulk-oriented drivers.) Gadget driver + responsibilities include: + + - handling setup requests (ep0 protocol responses) possibly + including class-specific functionality + + - returning configuration and string descriptors + + - (re)setting configurations and interface altsettings, including + enabling and configuring endpoints + + - handling life cycle events, such as managing bindings to + hardware, USB suspend/resume, remote wakeup, and disconnection + from the USB host. + + - managing IN and OUT transfers on all currently enabled endpoints + + Such drivers may be modules of proprietary code, although that + approach is discouraged in the Linux community. + +*Upper Level* + Most gadget drivers have an upper boundary that connects to some + Linux driver or framework in Linux. Through that boundary flows the + data which the gadget driver produces and/or consumes through + protocol transfers over USB. Examples include: + + - user mode code, using generic (gadgetfs) or application specific + files in ``/dev`` + + - networking subsystem (for network gadgets, like the CDC Ethernet + Model gadget driver) + + - data capture drivers, perhaps video4Linux or a scanner driver; or + test and measurement hardware. + + - input subsystem (for HID gadgets) + + - sound subsystem (for audio gadgets) + + - file system (for PTP gadgets) + + - block i/o subsystem (for usb-storage gadgets) + + - ... and more + +*Additional Layers* + Other layers may exist. These could include kernel layers, such as + network protocol stacks, as well as user mode applications building + on standard POSIX system call APIs such as *open()*, *close()*, + *read()* and *write()*. On newer systems, POSIX Async I/O calls may + be an option. Such user mode code will not necessarily be subject to + the GNU General Public License (GPL). + +OTG-capable systems will also need to include a standard Linux-USB host +side stack, with *usbcore*, one or more *Host Controller Drivers* +(HCDs), *USB Device Drivers* to support the OTG "Targeted Peripheral +List", and so forth. There will also be an *OTG Controller Driver*, +which is visible to gadget and device driver developers only indirectly. +That helps the host and device side USB controllers implement the two +new OTG protocols (HNP and SRP). Roles switch (host to peripheral, or +vice versa) using HNP during USB suspend processing, and SRP can be +viewed as a more battery-friendly kind of device wakeup protocol. + +Over time, reusable utilities are evolving to help make some gadget +driver tasks simpler. For example, building configuration descriptors +from vectors of descriptors for the configurations interfaces and +endpoints is now automated, and many drivers now use autoconfiguration +to choose hardware endpoints and initialize their descriptors. A +potential example of particular interest is code implementing standard +USB-IF protocols for HID, networking, storage, or audio classes. Some +developers are interested in KDB or KGDB hooks, to let target hardware +be remotely debugged. Most such USB protocol code doesn't need to be +hardware-specific, any more than network protocols like X11, HTTP, or +NFS are. Such gadget-side interface drivers should eventually be +combined, to implement composite devices. + +Kernel Mode Gadget API +====================== + +Gadget drivers declare themselves through a *struct +usb_gadget_driver*, which is responsible for most parts of enumeration +for a *struct usb_gadget*. The response to a set_configuration usually +involves enabling one or more of the *struct usb_ep* objects exposed by +the gadget, and submitting one or more *struct usb_request* buffers to +transfer data. Understand those four data types, and their operations, +and you will understand how this API works. + + **Note** + + This documentation was prepared using the standard Linux kernel + ``docproc`` tool, which turns text and in-code comments into SGML + DocBook and then into usable formats such as HTML or PDF. Other than + the "Chapter 9" data types, most of the significant data types and + functions are described here. + + However, docproc does not understand all the C constructs that are + used, so some relevant information is likely omitted from what you + are reading. One example of such information is endpoint + autoconfiguration. You'll have to read the header file, and use + example source code (such as that for "Gadget Zero"), to fully + understand the API. + + The part of the API implementing some basic driver capabilities is + specific to the version of the Linux kernel that's in use. The 2.6 + kernel includes a *driver model* framework that has no analogue on + earlier kernels; so those parts of the gadget API are not fully + portable. (They are implemented on 2.4 kernels, but in a different + way.) The driver model state is another part of this API that is + ignored by the kerneldoc tools. + +The core API does not expose every possible hardware feature, only the +most widely available ones. There are significant hardware features, +such as device-to-device DMA (without temporary storage in a memory +buffer) that would be added using hardware-specific APIs. + +This API allows drivers to use conditional compilation to handle +endpoint capabilities of different hardware, but doesn't require that. +Hardware tends to have arbitrary restrictions, relating to transfer +types, addressing, packet sizes, buffering, and availability. As a rule, +such differences only matter for "endpoint zero" logic that handles +device configuration and management. The API supports limited run-time +detection of capabilities, through naming conventions for endpoints. +Many drivers will be able to at least partially autoconfigure +themselves. In particular, driver init sections will often have endpoint +autoconfiguration logic that scans the hardware's list of endpoints to +find ones matching the driver requirements (relying on those +conventions), to eliminate some of the most common reasons for +conditional compilation. + +Like the Linux-USB host side API, this API exposes the "chunky" nature +of USB messages: I/O requests are in terms of one or more "packets", and +packet boundaries are visible to drivers. Compared to RS-232 serial +protocols, USB resembles synchronous protocols like HDLC (N bytes per +frame, multipoint addressing, host as the primary station and devices as +secondary stations) more than asynchronous ones (tty style: 8 data bits +per frame, no parity, one stop bit). So for example the controller +drivers won't buffer two single byte writes into a single two-byte USB +IN packet, although gadget drivers may do so when they implement +protocols where packet boundaries (and "short packets") are not +significant. + +Driver Life Cycle +----------------- + +Gadget drivers make endpoint I/O requests to hardware without needing to +know many details of the hardware, but driver setup/configuration code +needs to handle some differences. Use the API like this: + +1. Register a driver for the particular device side usb controller + hardware, such as the net2280 on PCI (USB 2.0), sa11x0 or pxa25x as + found in Linux PDAs, and so on. At this point the device is logically + in the USB ch9 initial state ("attached"), drawing no power and not + usable (since it does not yet support enumeration). Any host should + not see the device, since it's not activated the data line pullup + used by the host to detect a device, even if VBUS power is available. + +2. Register a gadget driver that implements some higher level device + function. That will then bind() to a usb_gadget, which activates the + data line pullup sometime after detecting VBUS. + +3. The hardware driver can now start enumerating. The steps it handles + are to accept USB power and set_address requests. Other steps are + handled by the gadget driver. If the gadget driver module is unloaded + before the host starts to enumerate, steps before step 7 are skipped. + +4. The gadget driver's setup() call returns usb descriptors, based both + on what the bus interface hardware provides and on the functionality + being implemented. That can involve alternate settings or + configurations, unless the hardware prevents such operation. For OTG + devices, each configuration descriptor includes an OTG descriptor. + +5. The gadget driver handles the last step of enumeration, when the USB + host issues a set_configuration call. It enables all endpoints used + in that configuration, with all interfaces in their default settings. + That involves using a list of the hardware's endpoints, enabling each + endpoint according to its descriptor. It may also involve using + :c:func:`usb_gadget_vbus_draw()` to let more power be drawn + from VBUS, as allowed by that configuration. For OTG devices, setting + a configuration may also involve reporting HNP capabilities through a + user interface. + +6. Do real work and perform data transfers, possibly involving changes + to interface settings or switching to new configurations, until the + device is disconnect()ed from the host. Queue any number of transfer + requests to each endpoint. It may be suspended and resumed several + times before being disconnected. On disconnect, the drivers go back + to step 3 (above). + +7. When the gadget driver module is being unloaded, the driver unbind() + callback is issued. That lets the controller driver be unloaded. + +Drivers will normally be arranged so that just loading the gadget driver +module (or statically linking it into a Linux kernel) allows the +peripheral device to be enumerated, but some drivers will defer +enumeration until some higher level component (like a user mode daemon) +enables it. Note that at this lowest level there are no policies about +how ep0 configuration logic is implemented, except that it should obey +USB specifications. Such issues are in the domain of gadget drivers, +including knowing about implementation constraints imposed by some USB +controllers or understanding that composite devices might happen to be +built by integrating reusable components. + +Note that the lifecycle above can be slightly different for OTG devices. +Other than providing an additional OTG descriptor in each configuration, +only the HNP-related differences are particularly visible to driver +code. They involve reporting requirements during the SET_CONFIGURATION +request, and the option to invoke HNP during some suspend callbacks. +Also, SRP changes the semantics of :c:func:`usb_gadget_wakeup()` +slightly. + +USB 2.0 Chapter 9 Types and Constants +------------------------------------- + +Gadget drivers rely on common USB structures and constants defined in +the ```` header file, which is standard in Linux 2.6 +kernels. These are the same types and constants used by host side +drivers (and usbcore). + +.. kernel-doc:: include/linux/usb/ch9.h + :internal: + +Core Objects and Methods +------------------------ + +These are declared in ````, and are used by gadget +drivers to interact with USB peripheral controller drivers. + +.. kernel-doc:: include/linux/usb/gadget.h + :internal: + +Optional Utilities +------------------ + +The core API is sufficient for writing a USB Gadget Driver, but some +optional utilities are provided to simplify common tasks. These +utilities include endpoint autoconfiguration. + +.. kernel-doc:: drivers/usb/gadget/usbstring.c + :export: + +.. kernel-doc:: drivers/usb/gadget/config.c + :export: + +Composite Device Framework +-------------------------- + +The core API is sufficient for writing drivers for composite USB devices +(with more than one function in a given configuration), and also +multi-configuration devices (also more than one function, but not +necessarily sharing a given configuration). There is however an optional +framework which makes it easier to reuse and combine functions. + +Devices using this framework provide a *struct usb_composite_driver*, +which in turn provides one or more *struct usb_configuration* +instances. Each such configuration includes at least one *struct +usb_function*, which packages a user visible role such as "network +link" or "mass storage device". Management functions may also exist, +such as "Device Firmware Upgrade". + +.. kernel-doc:: include/linux/usb/composite.h + :internal: + +.. kernel-doc:: drivers/usb/gadget/composite.c + :export: + +Composite Device Functions +-------------------------- + +At this writing, a few of the current gadget drivers have been converted +to this framework. Near-term plans include converting all of them, +except for "gadgetfs". + +.. kernel-doc:: drivers/usb/gadget/function/f_acm.c + :export: + +.. kernel-doc:: drivers/usb/gadget/function/f_ecm.c + :export: + +.. kernel-doc:: drivers/usb/gadget/function/f_subset.c + :export: + +.. kernel-doc:: drivers/usb/gadget/function/f_obex.c + :export: + +.. kernel-doc:: drivers/usb/gadget/function/f_serial.c + :export: + +Peripheral Controller Drivers +============================= + +The first hardware supporting this API was the NetChip 2280 controller, +which supports USB 2.0 high speed and is based on PCI. This is the +``net2280`` driver module. The driver supports Linux kernel versions 2.4 +and 2.6; contact NetChip Technologies for development boards and product +information. + +Other hardware working in the "gadget" framework includes: Intel's PXA +25x and IXP42x series processors (``pxa2xx_udc``), Toshiba TC86c001 +"Goku-S" (``goku_udc``), Renesas SH7705/7727 (``sh_udc``), MediaQ 11xx +(``mq11xx_udc``), Hynix HMS30C7202 (``h7202_udc``), National 9303/4 +(``n9604_udc``), Texas Instruments OMAP (``omap_udc``), Sharp LH7A40x +(``lh7a40x_udc``), and more. Most of those are full speed controllers. + +At this writing, there are people at work on drivers in this framework +for several other USB device controllers, with plans to make many of +them be widely available. + +A partial USB simulator, the ``dummy_hcd`` driver, is available. It can +act like a net2280, a pxa25x, or an sa11x0 in terms of available +endpoints and device speeds; and it simulates control, bulk, and to some +extent interrupt transfers. That lets you develop some parts of a gadget +driver on a normal PC, without any special hardware, and perhaps with +the assistance of tools such as GDB running with User Mode Linux. At +least one person has expressed interest in adapting that approach, +hooking it up to a simulator for a microcontroller. Such simulators can +help debug subsystems where the runtime hardware is unfriendly to +software development, or is not yet available. + +Support for other controllers is expected to be developed and +contributed over time, as this driver framework evolves. + +Gadget Drivers +============== + +In addition to *Gadget Zero* (used primarily for testing and development +with drivers for usb controller hardware), other gadget drivers exist. + +There's an *ethernet* gadget driver, which implements one of the most +useful *Communications Device Class* (CDC) models. One of the standards +for cable modem interoperability even specifies the use of this ethernet +model as one of two mandatory options. Gadgets using this code look to a +USB host as if they're an Ethernet adapter. It provides access to a +network where the gadget's CPU is one host, which could easily be +bridging, routing, or firewalling access to other networks. Since some +hardware can't fully implement the CDC Ethernet requirements, this +driver also implements a "good parts only" subset of CDC Ethernet. (That +subset doesn't advertise itself as CDC Ethernet, to avoid creating +problems.) + +Support for Microsoft's *RNDIS* protocol has been contributed by +Pengutronix and Auerswald GmbH. This is like CDC Ethernet, but it runs +on more slightly USB hardware (but less than the CDC subset). However, +its main claim to fame is being able to connect directly to recent +versions of Windows, using drivers that Microsoft bundles and supports, +making it much simpler to network with Windows. + +There is also support for user mode gadget drivers, using *gadgetfs*. +This provides a *User Mode API* that presents each endpoint as a single +file descriptor. I/O is done using normal *read()* and *read()* calls. +Familiar tools like GDB and pthreads can be used to develop and debug +user mode drivers, so that once a robust controller driver is available +many applications for it won't require new kernel mode software. Linux +2.6 *Async I/O (AIO)* support is available, so that user mode software +can stream data with only slightly more overhead than a kernel driver. + +There's a USB Mass Storage class driver, which provides a different +solution for interoperability with systems such as MS-Windows and MacOS. +That *Mass Storage* driver uses a file or block device as backing store +for a drive, like the ``loop`` driver. The USB host uses the BBB, CB, or +CBI versions of the mass storage class specification, using transparent +SCSI commands to access the data from the backing store. + +There's a "serial line" driver, useful for TTY style operation over USB. +The latest version of that driver supports CDC ACM style operation, like +a USB modem, and so on most hardware it can interoperate easily with +MS-Windows. One interesting use of that driver is in boot firmware (like +a BIOS), which can sometimes use that model with very small systems +without real serial lines. + +Support for other kinds of gadget is expected to be developed and +contributed over time, as this driver framework evolves. + +USB On-The-GO (OTG) +=================== + +USB OTG support on Linux 2.6 was initially developed by Texas +Instruments for `OMAP `__ 16xx and 17xx series +processors. Other OTG systems should work in similar ways, but the +hardware level details could be very different. + +Systems need specialized hardware support to implement OTG, notably +including a special *Mini-AB* jack and associated transceiver to support +*Dual-Role* operation: they can act either as a host, using the standard +Linux-USB host side driver stack, or as a peripheral, using this +"gadget" framework. To do that, the system software relies on small +additions to those programming interfaces, and on a new internal +component (here called an "OTG Controller") affecting which driver stack +connects to the OTG port. In each role, the system can re-use the +existing pool of hardware-neutral drivers, layered on top of the +controller driver interfaces (*usb_bus* or *usb_gadget*). Such drivers +need at most minor changes, and most of the calls added to support OTG +can also benefit non-OTG products. + +- Gadget drivers test the *is_otg* flag, and use it to determine + whether or not to include an OTG descriptor in each of their + configurations. + +- Gadget drivers may need changes to support the two new OTG protocols, + exposed in new gadget attributes such as *b_hnp_enable* flag. HNP + support should be reported through a user interface (two LEDs could + suffice), and is triggered in some cases when the host suspends the + peripheral. SRP support can be user-initiated just like remote + wakeup, probably by pressing the same button. + +- On the host side, USB device drivers need to be taught to trigger HNP + at appropriate moments, using :c:func:`usb_suspend_device()`. + That also conserves battery power, which is useful even for non-OTG + configurations. + +- Also on the host side, a driver must support the OTG "Targeted + Peripheral List". That's just a whitelist, used to reject peripherals + not supported with a given Linux OTG host. *This whitelist is + product-specific; each product must modify ``otg_whitelist.h`` to + match its interoperability specification.* + + Non-OTG Linux hosts, like PCs and workstations, normally have some + solution for adding drivers, so that peripherals that aren't + recognized can eventually be supported. That approach is unreasonable + for consumer products that may never have their firmware upgraded, + and where it's usually unrealistic to expect traditional + PC/workstation/server kinds of support model to work. For example, + it's often impractical to change device firmware once the product has + been distributed, so driver bugs can't normally be fixed if they're + found after shipment. + +Additional changes are needed below those hardware-neutral *usb_bus* +and *usb_gadget* driver interfaces; those aren't discussed here in any +detail. Those affect the hardware-specific code for each USB Host or +Peripheral controller, and how the HCD initializes (since OTG can be +active only on a single port). They also involve what may be called an +*OTG Controller Driver*, managing the OTG transceiver and the OTG state +machine logic as well as much of the root hub behavior for the OTG port. +The OTG controller driver needs to activate and deactivate USB +controllers depending on the relevant device role. Some related changes +were needed inside usbcore, so that it can identify OTG-capable devices +and respond appropriately to HNP or SRP protocols. diff --git a/Documentation/driver-api/usb/index.rst b/Documentation/driver-api/usb/index.rst new file mode 100644 index 000000000000..cf2fa2e8d236 --- /dev/null +++ b/Documentation/driver-api/usb/index.rst @@ -0,0 +1,17 @@ +============= +Linux USB API +============= + +.. toctree:: + + usb + gadget + writing_usb_driver + writing_musb_glue_layer + +.. only:: subproject and html + + Indices + ======= + + * :ref:`genindex` diff --git a/Documentation/driver-api/usb.rst b/Documentation/driver-api/usb/usb.rst similarity index 87% rename from Documentation/driver-api/usb.rst rename to Documentation/driver-api/usb/usb.rst index 851cc40b66b5..b856abb3200e 100644 --- a/Documentation/driver-api/usb.rst +++ b/Documentation/driver-api/usb/usb.rst @@ -329,11 +329,11 @@ to detect when devices are added or removed: fd = open("/proc/bus/usb/devices", O_RDONLY); pfd = { fd, POLLIN, 0 }; for (;;) { - /* The first time through, this call will return immediately. */ - poll(&pfd, 1, -1); + /* The first time through, this call will return immediately. */ + poll(&pfd, 1, -1); - /* To see what's changed, compare the file's previous and current - contents or scan the filesystem. (Scanning is more precise.) */ + /* To see what's changed, compare the file's previous and current + contents or scan the filesystem. (Scanning is more precise.) */ } Note that this behavior is intended to be used for informational and @@ -462,10 +462,10 @@ USBDEVFS_CONNECTINFO :: - struct usbdevfs_connectinfo { - unsigned int devnum; - unsigned char slow; - }; + struct usbdevfs_connectinfo { + unsigned int devnum; + unsigned char slow; + }; File modification time is not updated by this request. @@ -481,10 +481,10 @@ USBDEVFS_GETDRIVER :: - struct usbdevfs_getdriver { - unsigned int interface; - char driver[USBDEVFS_MAXDRIVERNAME + 1]; - }; + struct usbdevfs_getdriver { + unsigned int interface; + char driver[USBDEVFS_MAXDRIVERNAME + 1]; + }; File modification time is not updated by this request. @@ -494,28 +494,28 @@ USBDEVFS_IOCTL :: - struct usbdevfs_ioctl { - int ifno; - int ioctl_code; - void *data; - }; + struct usbdevfs_ioctl { + int ifno; + int ioctl_code; + void *data; + }; - /* user mode call looks like this. - * 'request' becomes the driver->ioctl() 'code' parameter. - * the size of 'param' is encoded in 'request', and that data - * is copied to or from the driver->ioctl() 'buf' parameter. - */ - static int - usbdev_ioctl (int fd, int ifno, unsigned request, void *param) - { - struct usbdevfs_ioctl wrapper; + /* user mode call looks like this. + * 'request' becomes the driver->ioctl() 'code' parameter. + * the size of 'param' is encoded in 'request', and that data + * is copied to or from the driver->ioctl() 'buf' parameter. + */ + static int + usbdev_ioctl (int fd, int ifno, unsigned request, void *param) + { + struct usbdevfs_ioctl wrapper; - wrapper.ifno = ifno; - wrapper.ioctl_code = request; - wrapper.data = param; + wrapper.ifno = ifno; + wrapper.ioctl_code = request; + wrapper.data = param; - return ioctl (fd, USBDEVFS_IOCTL, &wrapper); - } + return ioctl (fd, USBDEVFS_IOCTL, &wrapper); + } File modification time is not updated by this request. @@ -534,11 +534,11 @@ USBDEVFS_RELEASEINTERFACE the number of the interface (bInterfaceNumber from descriptor); File modification time is not updated by this request. - **Warning** + **Warning** - *No security check is made to ensure that the task which made - the claim is the one which is releasing it. This means that user - mode driver may interfere other ones.* + *No security check is made to ensure that the task which made + the claim is the one which is releasing it. This means that user + mode driver may interfere other ones.* USBDEVFS_RESETEP Resets the data toggle value for an endpoint (bulk or interrupt) to @@ -546,13 +546,13 @@ USBDEVFS_RESETEP as identified in the endpoint descriptor), with USB_DIR_IN added if the device's endpoint sends data to the host. - **Warning** + **Warning** - *Avoid using this request. It should probably be removed.* Using - it typically means the device and driver will lose toggle - synchronization. If you really lost synchronization, you likely - need to completely handshake with the device, using a request - like CLEAR_HALT or SET_INTERFACE. + *Avoid using this request. It should probably be removed.* Using + it typically means the device and driver will lose toggle + synchronization. If you really lost synchronization, you likely + need to completely handshake with the device, using a request + like CLEAR_HALT or SET_INTERFACE. USBDEVFS_DROP_PRIVILEGES This is used to relinquish the ability to do certain operations @@ -578,12 +578,12 @@ USBDEVFS_BULK :: - struct usbdevfs_bulktransfer { - unsigned int ep; - unsigned int len; - unsigned int timeout; /* in milliseconds */ - void *data; - }; + struct usbdevfs_bulktransfer { + unsigned int ep; + unsigned int len; + unsigned int timeout; /* in milliseconds */ + void *data; + }; The "ep" value identifies a bulk endpoint number (1 to 15, as identified in an endpoint descriptor), masked with USB_DIR_IN when @@ -610,15 +610,15 @@ USBDEVFS_CONTROL :: - struct usbdevfs_ctrltransfer { - __u8 bRequestType; - __u8 bRequest; - __u16 wValue; - __u16 wIndex; - __u16 wLength; - __u32 timeout; /* in milliseconds */ - void *data; - }; + struct usbdevfs_ctrltransfer { + __u8 bRequestType; + __u8 bRequest; + __u16 wValue; + __u16 wIndex; + __u16 wLength; + __u32 timeout; /* in milliseconds */ + void *data; + }; The first eight bytes of this structure are the contents of the SETUP packet to be sent to the device; see the USB 2.0 specification @@ -638,11 +638,11 @@ USBDEVFS_RESET the reset, this rebinds all device interfaces. File modification time is not updated by this request. - **Warning** + **Warning** - *Avoid using this call* until some usbcore bugs get fixed, since - it does not fully synchronize device, interface, and driver (not - just usbfs) state. + *Avoid using this call* until some usbcore bugs get fixed, since + it does not fully synchronize device, interface, and driver (not + just usbfs) state. USBDEVFS_SETINTERFACE Sets the alternate setting for an interface. The ioctl parameter is @@ -650,10 +650,10 @@ USBDEVFS_SETINTERFACE :: - struct usbdevfs_setinterface { - unsigned int interface; - unsigned int altsetting; - }; + struct usbdevfs_setinterface { + unsigned int interface; + unsigned int altsetting; + }; File modification time is not updated by this request. @@ -669,11 +669,11 @@ USBDEVFS_SETCONFIGURATION configuration (bConfigurationValue from descriptor). File modification time is not updated by this request. - **Warning** + **Warning** - *Avoid using this call* until some usbcore bugs get fixed, since - it does not fully synchronize device, interface, and driver (not - just usbfs) state. + *Avoid using this call* until some usbcore bugs get fixed, since + it does not fully synchronize device, interface, and driver (not + just usbfs) state. Asynchronous I/O Support ~~~~~~~~~~~~~~~~~~~~~~~~ @@ -707,25 +707,25 @@ fewer bytes were read than were requested then you get an error report. :: struct usbdevfs_iso_packet_desc { - unsigned int length; - unsigned int actual_length; - unsigned int status; + unsigned int length; + unsigned int actual_length; + unsigned int status; }; struct usbdevfs_urb { - unsigned char type; - unsigned char endpoint; - int status; - unsigned int flags; - void *buffer; - int buffer_length; - int actual_length; - int start_frame; - int number_of_packets; - int error_count; - unsigned int signr; - void *usercontext; - struct usbdevfs_iso_packet_desc iso_frame_desc[]; + unsigned char type; + unsigned char endpoint; + int status; + unsigned int flags; + void *buffer; + int buffer_length; + int actual_length; + int start_frame; + int number_of_packets; + int error_count; + unsigned int signr; + void *usercontext; + struct usbdevfs_iso_packet_desc iso_frame_desc[]; }; For these asynchronous requests, the file modification time reflects diff --git a/Documentation/driver-api/usb/writing_musb_glue_layer.rst b/Documentation/driver-api/usb/writing_musb_glue_layer.rst new file mode 100644 index 000000000000..52700c7031f9 --- /dev/null +++ b/Documentation/driver-api/usb/writing_musb_glue_layer.rst @@ -0,0 +1,737 @@ +========================== +Writing an MUSB Glue Layer +========================== + +:Author: Apelete Seketeli + +Introduction +============ + +The Linux MUSB subsystem is part of the larger Linux USB subsystem. It +provides support for embedded USB Device Controllers (UDC) that do not +use Universal Host Controller Interface (UHCI) or Open Host Controller +Interface (OHCI). + +Instead, these embedded UDC rely on the USB On-the-Go (OTG) +specification which they implement at least partially. The silicon +reference design used in most cases is the Multipoint USB Highspeed +Dual-Role Controller (MUSB HDRC) found in the Mentor Graphics Inventra™ +design. + +As a self-taught exercise I have written an MUSB glue layer for the +Ingenic JZ4740 SoC, modelled after the many MUSB glue layers in the +kernel source tree. This layer can be found at +drivers/usb/musb/jz4740.c. In this documentation I will walk through the +basics of the jz4740.c glue layer, explaining the different pieces and +what needs to be done in order to write your own device glue layer. + +Linux MUSB Basics +================= + +To get started on the topic, please read USB On-the-Go Basics (see +Resources) which provides an introduction of USB OTG operation at the +hardware level. A couple of wiki pages by Texas Instruments and Analog +Devices also provide an overview of the Linux kernel MUSB configuration, +albeit focused on some specific devices provided by these companies. +Finally, getting acquainted with the USB specification at USB home page +may come in handy, with practical instance provided through the Writing +USB Device Drivers documentation (again, see Resources). + +Linux USB stack is a layered architecture in which the MUSB controller +hardware sits at the lowest. The MUSB controller driver abstract the +MUSB controller hardware to the Linux USB stack. + +:: + + ------------------------ + | | <------- drivers/usb/gadget + | Linux USB Core Stack | <------- drivers/usb/host + | | <------- drivers/usb/core + ------------------------ + ⬍ + -------------------------- + | | <------ drivers/usb/musb/musb_gadget.c + | MUSB Controller driver | <------ drivers/usb/musb/musb_host.c + | | <------ drivers/usb/musb/musb_core.c + -------------------------- + ⬍ + --------------------------------- + | MUSB Platform Specific Driver | + | | <-- drivers/usb/musb/jz4740.c + | aka "Glue Layer" | + --------------------------------- + ⬍ + --------------------------------- + | MUSB Controller Hardware | + --------------------------------- + + +As outlined above, the glue layer is actually the platform specific code +sitting in between the controller driver and the controller hardware. + +Just like a Linux USB driver needs to register itself with the Linux USB +subsystem, the MUSB glue layer needs first to register itself with the +MUSB controller driver. This will allow the controller driver to know +about which device the glue layer supports and which functions to call +when a supported device is detected or released; remember we are talking +about an embedded controller chip here, so no insertion or removal at +run-time. + +All of this information is passed to the MUSB controller driver through +a platform_driver structure defined in the glue layer as: + +:: + + static struct platform_driver jz4740_driver = { + .probe = jz4740_probe, + .remove = jz4740_remove, + .driver = { + .name = "musb-jz4740", + }, + }; + + +The probe and remove function pointers are called when a matching device +is detected and, respectively, released. The name string describes the +device supported by this glue layer. In the current case it matches a +platform_device structure declared in arch/mips/jz4740/platform.c. Note +that we are not using device tree bindings here. + +In order to register itself to the controller driver, the glue layer +goes through a few steps, basically allocating the controller hardware +resources and initialising a couple of circuits. To do so, it needs to +keep track of the information used throughout these steps. This is done +by defining a private jz4740_glue structure: + +:: + + struct jz4740_glue { + struct device *dev; + struct platform_device *musb; + struct clk *clk; + }; + + +The dev and musb members are both device structure variables. The first +one holds generic information about the device, since it's the basic +device structure, and the latter holds information more closely related +to the subsystem the device is registered to. The clk variable keeps +information related to the device clock operation. + +Let's go through the steps of the probe function that leads the glue +layer to register itself to the controller driver. + +N.B.: For the sake of readability each function will be split in logical +parts, each part being shown as if it was independent from the others. + +:: + + static int jz4740_probe(struct platform_device *pdev) + { + struct platform_device *musb; + struct jz4740_glue *glue; + struct clk *clk; + int ret; + + glue = devm_kzalloc(&pdev->dev, sizeof(*glue), GFP_KERNEL); + if (!glue) + return -ENOMEM; + + musb = platform_device_alloc("musb-hdrc", PLATFORM_DEVID_AUTO); + if (!musb) { + dev_err(&pdev->dev, "failed to allocate musb device\n"); + return -ENOMEM; + } + + clk = devm_clk_get(&pdev->dev, "udc"); + if (IS_ERR(clk)) { + dev_err(&pdev->dev, "failed to get clock\n"); + ret = PTR_ERR(clk); + goto err_platform_device_put; + } + + ret = clk_prepare_enable(clk); + if (ret) { + dev_err(&pdev->dev, "failed to enable clock\n"); + goto err_platform_device_put; + } + + musb->dev.parent = &pdev->dev; + + glue->dev = &pdev->dev; + glue->musb = musb; + glue->clk = clk; + + return 0; + + err_platform_device_put: + platform_device_put(musb); + return ret; + } + + +The first few lines of the probe function allocate and assign the glue, +musb and clk variables. The GFP_KERNEL flag (line 8) allows the +allocation process to sleep and wait for memory, thus being usable in a +blocking situation. The PLATFORM_DEVID_AUTO flag (line 12) allows +automatic allocation and management of device IDs in order to avoid +device namespace collisions with explicit IDs. With devm_clk_get() +(line 18) the glue layer allocates the clock -- the ``devm_`` prefix +indicates that clk_get() is managed: it automatically frees the +allocated clock resource data when the device is released -- and enable +it. + +Then comes the registration steps: + +:: + + static int jz4740_probe(struct platform_device *pdev) + { + struct musb_hdrc_platform_data *pdata = &jz4740_musb_platform_data; + + pdata->platform_ops = &jz4740_musb_ops; + + platform_set_drvdata(pdev, glue); + + ret = platform_device_add_resources(musb, pdev->resource, + pdev->num_resources); + if (ret) { + dev_err(&pdev->dev, "failed to add resources\n"); + goto err_clk_disable; + } + + ret = platform_device_add_data(musb, pdata, sizeof(*pdata)); + if (ret) { + dev_err(&pdev->dev, "failed to add platform_data\n"); + goto err_clk_disable; + } + + return 0; + + err_clk_disable: + clk_disable_unprepare(clk); + err_platform_device_put: + platform_device_put(musb); + return ret; + } + + +The first step is to pass the device data privately held by the glue +layer on to the controller driver through platform_set_drvdata() (line +7). Next is passing on the device resources information, also privately +held at that point, through platform_device_add_resources() (line 9). + +Finally comes passing on the platform specific data to the controller +driver (line 16). Platform data will be discussed in `Chapter +4 <#device-platform-data>`__, but here we are looking at the +platform_ops function pointer (line 5) in musb_hdrc_platform_data +structure (line 3). This function pointer allows the MUSB controller +driver to know which function to call for device operation: + +:: + + static const struct musb_platform_ops jz4740_musb_ops = { + .init = jz4740_musb_init, + .exit = jz4740_musb_exit, + }; + + +Here we have the minimal case where only init and exit functions are +called by the controller driver when needed. Fact is the JZ4740 MUSB +controller is a basic controller, lacking some features found in other +controllers, otherwise we may also have pointers to a few other +functions like a power management function or a function to switch +between OTG and non-OTG modes, for instance. + +At that point of the registration process, the controller driver +actually calls the init function: + +:: + + static int jz4740_musb_init(struct musb *musb) + { + musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2); + if (!musb->xceiv) { + pr_err("HS UDC: no transceiver configured\n"); + return -ENODEV; + } + + /* Silicon does not implement ConfigData register. + * Set dyn_fifo to avoid reading EP config from hardware. + */ + musb->dyn_fifo = true; + + musb->isr = jz4740_musb_interrupt; + + return 0; + } + + +The goal of jz4740_musb_init() is to get hold of the transceiver +driver data of the MUSB controller hardware and pass it on to the MUSB +controller driver, as usual. The transceiver is the circuitry inside the +controller hardware responsible for sending/receiving the USB data. +Since it is an implementation of the physical layer of the OSI model, +the transceiver is also referred to as PHY. + +Getting hold of the MUSB PHY driver data is done with usb_get_phy() +which returns a pointer to the structure containing the driver instance +data. The next couple of instructions (line 12 and 14) are used as a +quirk and to setup IRQ handling respectively. Quirks and IRQ handling +will be discussed later in `Chapter 5 <#device-quirks>`__ and `Chapter +3 <#handling-irqs>`__. + +:: + + static int jz4740_musb_exit(struct musb *musb) + { + usb_put_phy(musb->xceiv); + + return 0; + } + + +Acting as the counterpart of init, the exit function releases the MUSB +PHY driver when the controller hardware itself is about to be released. + +Again, note that init and exit are fairly simple in this case due to the +basic set of features of the JZ4740 controller hardware. When writing an +musb glue layer for a more complex controller hardware, you might need +to take care of more processing in those two functions. + +Returning from the init function, the MUSB controller driver jumps back +into the probe function: + +:: + + static int jz4740_probe(struct platform_device *pdev) + { + ret = platform_device_add(musb); + if (ret) { + dev_err(&pdev->dev, "failed to register musb device\n"); + goto err_clk_disable; + } + + return 0; + + err_clk_disable: + clk_disable_unprepare(clk); + err_platform_device_put: + platform_device_put(musb); + return ret; + } + + +This is the last part of the device registration process where the glue +layer adds the controller hardware device to Linux kernel device +hierarchy: at this stage, all known information about the device is +passed on to the Linux USB core stack. + +:: + + static int jz4740_remove(struct platform_device *pdev) + { + struct jz4740_glue *glue = platform_get_drvdata(pdev); + + platform_device_unregister(glue->musb); + clk_disable_unprepare(glue->clk); + + return 0; + } + + +Acting as the counterpart of probe, the remove function unregister the +MUSB controller hardware (line 5) and disable the clock (line 6), +allowing it to be gated. + +Handling IRQs +============= + +Additionally to the MUSB controller hardware basic setup and +registration, the glue layer is also responsible for handling the IRQs: + +:: + + static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci) + { + unsigned long flags; + irqreturn_t retval = IRQ_NONE; + struct musb *musb = __hci; + + spin_lock_irqsave(&musb->lock, flags); + + musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB); + musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX); + musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX); + + /* + * The controller is gadget only, the state of the host mode IRQ bits is + * undefined. Mask them to make sure that the musb driver core will + * never see them set + */ + musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME | + MUSB_INTR_RESET | MUSB_INTR_SOF; + + if (musb->int_usb || musb->int_tx || musb->int_rx) + retval = musb_interrupt(musb); + + spin_unlock_irqrestore(&musb->lock, flags); + + return retval; + } + + +Here the glue layer mostly has to read the relevant hardware registers +and pass their values on to the controller driver which will handle the +actual event that triggered the IRQ. + +The interrupt handler critical section is protected by the +spin_lock_irqsave() and counterpart spin_unlock_irqrestore() +functions (line 7 and 24 respectively), which prevent the interrupt +handler code to be run by two different threads at the same time. + +Then the relevant interrupt registers are read (line 9 to 11): + +- MUSB_INTRUSB: indicates which USB interrupts are currently active, + +- MUSB_INTRTX: indicates which of the interrupts for TX endpoints are + currently active, + +- MUSB_INTRRX: indicates which of the interrupts for TX endpoints are + currently active. + +Note that musb_readb() is used to read 8-bit registers at most, while +musb_readw() allows us to read at most 16-bit registers. There are +other functions that can be used depending on the size of your device +registers. See musb_io.h for more information. + +Instruction on line 18 is another quirk specific to the JZ4740 USB +device controller, which will be discussed later in `Chapter +5 <#device-quirks>`__. + +The glue layer still needs to register the IRQ handler though. Remember +the instruction on line 14 of the init function: + +:: + + static int jz4740_musb_init(struct musb *musb) + { + musb->isr = jz4740_musb_interrupt; + + return 0; + } + + +This instruction sets a pointer to the glue layer IRQ handler function, +in order for the controller hardware to call the handler back when an +IRQ comes from the controller hardware. The interrupt handler is now +implemented and registered. + +Device Platform Data +==================== + +In order to write an MUSB glue layer, you need to have some data +describing the hardware capabilities of your controller hardware, which +is called the platform data. + +Platform data is specific to your hardware, though it may cover a broad +range of devices, and is generally found somewhere in the arch/ +directory, depending on your device architecture. + +For instance, platform data for the JZ4740 SoC is found in +arch/mips/jz4740/platform.c. In the platform.c file each device of the +JZ4740 SoC is described through a set of structures. + +Here is the part of arch/mips/jz4740/platform.c that covers the USB +Device Controller (UDC): + +:: + + /* USB Device Controller */ + struct platform_device jz4740_udc_xceiv_device = { + .name = "usb_phy_gen_xceiv", + .id = 0, + }; + + static struct resource jz4740_udc_resources[] = { + [0] = { + .start = JZ4740_UDC_BASE_ADDR, + .end = JZ4740_UDC_BASE_ADDR + 0x10000 - 1, + .flags = IORESOURCE_MEM, + }, + [1] = { + .start = JZ4740_IRQ_UDC, + .end = JZ4740_IRQ_UDC, + .flags = IORESOURCE_IRQ, + .name = "mc", + }, + }; + + struct platform_device jz4740_udc_device = { + .name = "musb-jz4740", + .id = -1, + .dev = { + .dma_mask = &jz4740_udc_device.dev.coherent_dma_mask, + .coherent_dma_mask = DMA_BIT_MASK(32), + }, + .num_resources = ARRAY_SIZE(jz4740_udc_resources), + .resource = jz4740_udc_resources, + }; + + +The jz4740_udc_xceiv_device platform device structure (line 2) +describes the UDC transceiver with a name and id number. + +At the time of this writing, note that "usb_phy_gen_xceiv" is the +specific name to be used for all transceivers that are either built-in +with reference USB IP or autonomous and doesn't require any PHY +programming. You will need to set CONFIG_NOP_USB_XCEIV=y in the +kernel configuration to make use of the corresponding transceiver +driver. The id field could be set to -1 (equivalent to +PLATFORM_DEVID_NONE), -2 (equivalent to PLATFORM_DEVID_AUTO) or +start with 0 for the first device of this kind if we want a specific id +number. + +The jz4740_udc_resources resource structure (line 7) defines the UDC +registers base addresses. + +The first array (line 9 to 11) defines the UDC registers base memory +addresses: start points to the first register memory address, end points +to the last register memory address and the flags member defines the +type of resource we are dealing with. So IORESOURCE_MEM is used to +define the registers memory addresses. The second array (line 14 to 17) +defines the UDC IRQ registers addresses. Since there is only one IRQ +register available for the JZ4740 UDC, start and end point at the same +address. The IORESOURCE_IRQ flag tells that we are dealing with IRQ +resources, and the name "mc" is in fact hard-coded in the MUSB core in +order for the controller driver to retrieve this IRQ resource by +querying it by its name. + +Finally, the jz4740_udc_device platform device structure (line 21) +describes the UDC itself. + +The "musb-jz4740" name (line 22) defines the MUSB driver that is used +for this device; remember this is in fact the name that we used in the +jz4740_driver platform driver structure in `Chapter +2 <#linux-musb-basics>`__. The id field (line 23) is set to -1 +(equivalent to PLATFORM_DEVID_NONE) since we do not need an id for the +device: the MUSB controller driver was already set to allocate an +automatic id in `Chapter 2 <#linux-musb-basics>`__. In the dev field we +care for DMA related information here. The dma_mask field (line 25) +defines the width of the DMA mask that is going to be used, and +coherent_dma_mask (line 26) has the same purpose but for the +alloc_coherent DMA mappings: in both cases we are using a 32 bits mask. +Then the resource field (line 29) is simply a pointer to the resource +structure defined before, while the num_resources field (line 28) keeps +track of the number of arrays defined in the resource structure (in this +case there were two resource arrays defined before). + +With this quick overview of the UDC platform data at the arch/ level now +done, let's get back to the MUSB glue layer specific platform data in +drivers/usb/musb/jz4740.c: + +:: + + static struct musb_hdrc_config jz4740_musb_config = { + /* Silicon does not implement USB OTG. */ + .multipoint = 0, + /* Max EPs scanned, driver will decide which EP can be used. */ + .num_eps = 4, + /* RAMbits needed to configure EPs from table */ + .ram_bits = 9, + .fifo_cfg = jz4740_musb_fifo_cfg, + .fifo_cfg_size = ARRAY_SIZE(jz4740_musb_fifo_cfg), + }; + + static struct musb_hdrc_platform_data jz4740_musb_platform_data = { + .mode = MUSB_PERIPHERAL, + .config = &jz4740_musb_config, + }; + + +First the glue layer configures some aspects of the controller driver +operation related to the controller hardware specifics. This is done +through the jz4740_musb_config musb_hdrc_config structure. + +Defining the OTG capability of the controller hardware, the multipoint +member (line 3) is set to 0 (equivalent to false) since the JZ4740 UDC +is not OTG compatible. Then num_eps (line 5) defines the number of USB +endpoints of the controller hardware, including endpoint 0: here we have +3 endpoints + endpoint 0. Next is ram_bits (line 7) which is the width +of the RAM address bus for the MUSB controller hardware. This +information is needed when the controller driver cannot automatically +configure endpoints by reading the relevant controller hardware +registers. This issue will be discussed when we get to device quirks in +`Chapter 5 <#device-quirks>`__. Last two fields (line 8 and 9) are also +about device quirks: fifo_cfg points to the USB endpoints configuration +table and fifo_cfg_size keeps track of the size of the number of +entries in that configuration table. More on that later in `Chapter +5 <#device-quirks>`__. + +Then this configuration is embedded inside jz4740_musb_platform_data +musb_hdrc_platform_data structure (line 11): config is a pointer to +the configuration structure itself, and mode tells the controller driver +if the controller hardware may be used as MUSB_HOST only, +MUSB_PERIPHERAL only or MUSB_OTG which is a dual mode. + +Remember that jz4740_musb_platform_data is then used to convey +platform data information as we have seen in the probe function in +`Chapter 2 <#linux-musb-basics>`__ + +Device Quirks +============= + +Completing the platform data specific to your device, you may also need +to write some code in the glue layer to work around some device specific +limitations. These quirks may be due to some hardware bugs, or simply be +the result of an incomplete implementation of the USB On-the-Go +specification. + +The JZ4740 UDC exhibits such quirks, some of which we will discuss here +for the sake of insight even though these might not be found in the +controller hardware you are working on. + +Let's get back to the init function first: + +:: + + static int jz4740_musb_init(struct musb *musb) + { + musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2); + if (!musb->xceiv) { + pr_err("HS UDC: no transceiver configured\n"); + return -ENODEV; + } + + /* Silicon does not implement ConfigData register. + * Set dyn_fifo to avoid reading EP config from hardware. + */ + musb->dyn_fifo = true; + + musb->isr = jz4740_musb_interrupt; + + return 0; + } + + +Instruction on line 12 helps the MUSB controller driver to work around +the fact that the controller hardware is missing registers that are used +for USB endpoints configuration. + +Without these registers, the controller driver is unable to read the +endpoints configuration from the hardware, so we use line 12 instruction +to bypass reading the configuration from silicon, and rely on a +hard-coded table that describes the endpoints configuration instead: + +:: + + static struct musb_fifo_cfg jz4740_musb_fifo_cfg[] = { + { .hw_ep_num = 1, .style = FIFO_TX, .maxpacket = 512, }, + { .hw_ep_num = 1, .style = FIFO_RX, .maxpacket = 512, }, + { .hw_ep_num = 2, .style = FIFO_TX, .maxpacket = 64, }, + }; + + +Looking at the configuration table above, we see that each endpoints is +described by three fields: hw_ep_num is the endpoint number, style is +its direction (either FIFO_TX for the controller driver to send packets +in the controller hardware, or FIFO_RX to receive packets from +hardware), and maxpacket defines the maximum size of each data packet +that can be transmitted over that endpoint. Reading from the table, the +controller driver knows that endpoint 1 can be used to send and receive +USB data packets of 512 bytes at once (this is in fact a bulk in/out +endpoint), and endpoint 2 can be used to send data packets of 64 bytes +at once (this is in fact an interrupt endpoint). + +Note that there is no information about endpoint 0 here: that one is +implemented by default in every silicon design, with a predefined +configuration according to the USB specification. For more examples of +endpoint configuration tables, see musb_core.c. + +Let's now get back to the interrupt handler function: + +:: + + static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci) + { + unsigned long flags; + irqreturn_t retval = IRQ_NONE; + struct musb *musb = __hci; + + spin_lock_irqsave(&musb->lock, flags); + + musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB); + musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX); + musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX); + + /* + * The controller is gadget only, the state of the host mode IRQ bits is + * undefined. Mask them to make sure that the musb driver core will + * never see them set + */ + musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME | + MUSB_INTR_RESET | MUSB_INTR_SOF; + + if (musb->int_usb || musb->int_tx || musb->int_rx) + retval = musb_interrupt(musb); + + spin_unlock_irqrestore(&musb->lock, flags); + + return retval; + } + + +Instruction on line 18 above is a way for the controller driver to work +around the fact that some interrupt bits used for USB host mode +operation are missing in the MUSB_INTRUSB register, thus left in an +undefined hardware state, since this MUSB controller hardware is used in +peripheral mode only. As a consequence, the glue layer masks these +missing bits out to avoid parasite interrupts by doing a logical AND +operation between the value read from MUSB_INTRUSB and the bits that +are actually implemented in the register. + +These are only a couple of the quirks found in the JZ4740 USB device +controller. Some others were directly addressed in the MUSB core since +the fixes were generic enough to provide a better handling of the issues +for others controller hardware eventually. + +Conclusion +========== + +Writing a Linux MUSB glue layer should be a more accessible task, as +this documentation tries to show the ins and outs of this exercise. + +The JZ4740 USB device controller being fairly simple, I hope its glue +layer serves as a good example for the curious mind. Used with the +current MUSB glue layers, this documentation should provide enough +guidance to get started; should anything gets out of hand, the linux-usb +mailing list archive is another helpful resource to browse through. + +Acknowledgements +================ + +Many thanks to Lars-Peter Clausen and Maarten ter Huurne for answering +my questions while I was writing the JZ4740 glue layer and for helping +me out getting the code in good shape. + +I would also like to thank the Qi-Hardware community at large for its +cheerful guidance and support. + +Resources +========= + +USB Home Page: http://www.usb.org + +linux-usb Mailing List Archives: http://marc.info/?l=linux-usb + +USB On-the-Go Basics: +http://www.maximintegrated.com/app-notes/index.mvp/id/1822 + +Writing USB Device Drivers: +https://www.kernel.org/doc/htmldocs/writing_usb_driver/index.html + +Texas Instruments USB Configuration Wiki Page: +http://processors.wiki.ti.com/index.php/Usbgeneralpage + +Analog Devices Blackfin MUSB Configuration: +http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:musb diff --git a/Documentation/driver-api/usb/writing_usb_driver.rst b/Documentation/driver-api/usb/writing_usb_driver.rst new file mode 100644 index 000000000000..c18dbd74152b --- /dev/null +++ b/Documentation/driver-api/usb/writing_usb_driver.rst @@ -0,0 +1,344 @@ +========================== +Writing USB Device Drivers +========================== + +:Author: Greg Kroah-Hartman + +Introduction +============ + +The Linux USB subsystem has grown from supporting only two different +types of devices in the 2.2.7 kernel (mice and keyboards), to over 20 +different types of devices in the 2.4 kernel. Linux currently supports +almost all USB class devices (standard types of devices like keyboards, +mice, modems, printers and speakers) and an ever-growing number of +vendor-specific devices (such as USB to serial converters, digital +cameras, Ethernet devices and MP3 players). For a full list of the +different USB devices currently supported, see Resources. + +The remaining kinds of USB devices that do not have support on Linux are +almost all vendor-specific devices. Each vendor decides to implement a +custom protocol to talk to their device, so a custom driver usually +needs to be created. Some vendors are open with their USB protocols and +help with the creation of Linux drivers, while others do not publish +them, and developers are forced to reverse-engineer. See Resources for +some links to handy reverse-engineering tools. + +Because each different protocol causes a new driver to be created, I +have written a generic USB driver skeleton, modelled after the +pci-skeleton.c file in the kernel source tree upon which many PCI +network drivers have been based. This USB skeleton can be found at +drivers/usb/usb-skeleton.c in the kernel source tree. In this article I +will walk through the basics of the skeleton driver, explaining the +different pieces and what needs to be done to customize it to your +specific device. + +Linux USB Basics +================ + +If you are going to write a Linux USB driver, please become familiar +with the USB protocol specification. It can be found, along with many +other useful documents, at the USB home page (see Resources). An +excellent introduction to the Linux USB subsystem can be found at the +USB Working Devices List (see Resources). It explains how the Linux USB +subsystem is structured and introduces the reader to the concept of USB +urbs (USB Request Blocks), which are essential to USB drivers. + +The first thing a Linux USB driver needs to do is register itself with +the Linux USB subsystem, giving it some information about which devices +the driver supports and which functions to call when a device supported +by the driver is inserted or removed from the system. All of this +information is passed to the USB subsystem in the usb_driver structure. +The skeleton driver declares a usb_driver as: + +:: + + static struct usb_driver skel_driver = { + .name = "skeleton", + .probe = skel_probe, + .disconnect = skel_disconnect, + .fops = &skel_fops, + .minor = USB_SKEL_MINOR_BASE, + .id_table = skel_table, + }; + + +The variable name is a string that describes the driver. It is used in +informational messages printed to the system log. The probe and +disconnect function pointers are called when a device that matches the +information provided in the id_table variable is either seen or +removed. + +The fops and minor variables are optional. Most USB drivers hook into +another kernel subsystem, such as the SCSI, network or TTY subsystem. +These types of drivers register themselves with the other kernel +subsystem, and any user-space interactions are provided through that +interface. But for drivers that do not have a matching kernel subsystem, +such as MP3 players or scanners, a method of interacting with user space +is needed. The USB subsystem provides a way to register a minor device +number and a set of file_operations function pointers that enable this +user-space interaction. The skeleton driver needs this kind of +interface, so it provides a minor starting number and a pointer to its +file_operations functions. + +The USB driver is then registered with a call to usb_register, usually +in the driver's init function, as shown here: + +:: + + static int __init usb_skel_init(void) + { + int result; + + /* register this driver with the USB subsystem */ + result = usb_register(&skel_driver); + if (result < 0) { + err("usb_register failed for the "__FILE__ "driver." + "Error number %d", result); + return -1; + } + + return 0; + } + module_init(usb_skel_init); + + +When the driver is unloaded from the system, it needs to deregister +itself with the USB subsystem. This is done with the usb_deregister +function: + +:: + + static void __exit usb_skel_exit(void) + { + /* deregister this driver with the USB subsystem */ + usb_deregister(&skel_driver); + } + module_exit(usb_skel_exit); + + +To enable the linux-hotplug system to load the driver automatically when +the device is plugged in, you need to create a MODULE_DEVICE_TABLE. +The following code tells the hotplug scripts that this module supports a +single device with a specific vendor and product ID: + +:: + + /* table of devices that work with this driver */ + static struct usb_device_id skel_table [] = { + { USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) }, + { } /* Terminating entry */ + }; + MODULE_DEVICE_TABLE (usb, skel_table); + + +There are other macros that can be used in describing a usb_device_id +for drivers that support a whole class of USB drivers. See usb.h for +more information on this. + +Device operation +================ + +When a device is plugged into the USB bus that matches the device ID +pattern that your driver registered with the USB core, the probe +function is called. The usb_device structure, interface number and the +interface ID are passed to the function: + +:: + + static int skel_probe(struct usb_interface *interface, + const struct usb_device_id *id) + + +The driver now needs to verify that this device is actually one that it +can accept. If so, it returns 0. If not, or if any error occurs during +initialization, an errorcode (such as ``-ENOMEM`` or ``-ENODEV``) is +returned from the probe function. + +In the skeleton driver, we determine what end points are marked as +bulk-in and bulk-out. We create buffers to hold the data that will be +sent and received from the device, and a USB urb to write data to the +device is initialized. + +Conversely, when the device is removed from the USB bus, the disconnect +function is called with the device pointer. The driver needs to clean +any private data that has been allocated at this time and to shut down +any pending urbs that are in the USB system. + +Now that the device is plugged into the system and the driver is bound +to the device, any of the functions in the file_operations structure +that were passed to the USB subsystem will be called from a user program +trying to talk to the device. The first function called will be open, as +the program tries to open the device for I/O. We increment our private +usage count and save a pointer to our internal structure in the file +structure. This is done so that future calls to file operations will +enable the driver to determine which device the user is addressing. All +of this is done with the following code: + +:: + + /* increment our usage count for the module */ + ++skel->open_count; + + /* save our object in the file's private structure */ + file->private_data = dev; + + +After the open function is called, the read and write functions are +called to receive and send data to the device. In the skel_write +function, we receive a pointer to some data that the user wants to send +to the device and the size of the data. The function determines how much +data it can send to the device based on the size of the write urb it has +created (this size depends on the size of the bulk out end point that +the device has). Then it copies the data from user space to kernel +space, points the urb to the data and submits the urb to the USB +subsystem. This can be seen in the following code: + +:: + + /* we can only write as much as 1 urb will hold */ + bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count; + + /* copy the data from user space into our urb */ + copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written); + + /* set up our urb */ + usb_fill_bulk_urb(skel->write_urb, + skel->dev, + usb_sndbulkpipe(skel->dev, skel->bulk_out_endpointAddr), + skel->write_urb->transfer_buffer, + bytes_written, + skel_write_bulk_callback, + skel); + + /* send the data out the bulk port */ + result = usb_submit_urb(skel->write_urb); + if (result) { + err("Failed submitting write urb, error %d", result); + } + + +When the write urb is filled up with the proper information using the +usb_fill_bulk_urb function, we point the urb's completion callback to +call our own skel_write_bulk_callback function. This function is +called when the urb is finished by the USB subsystem. The callback +function is called in interrupt context, so caution must be taken not to +do very much processing at that time. Our implementation of +skel_write_bulk_callback merely reports if the urb was completed +successfully or not and then returns. + +The read function works a bit differently from the write function in +that we do not use an urb to transfer data from the device to the +driver. Instead we call the usb_bulk_msg function, which can be used +to send or receive data from a device without having to create urbs and +handle urb completion callback functions. We call the usb_bulk_msg +function, giving it a buffer into which to place any data received from +the device and a timeout value. If the timeout period expires without +receiving any data from the device, the function will fail and return an +error message. This can be shown with the following code: + +:: + + /* do an immediate bulk read to get data from the device */ + retval = usb_bulk_msg (skel->dev, + usb_rcvbulkpipe (skel->dev, + skel->bulk_in_endpointAddr), + skel->bulk_in_buffer, + skel->bulk_in_size, + &count, HZ*10); + /* if the read was successful, copy the data to user space */ + if (!retval) { + if (copy_to_user (buffer, skel->bulk_in_buffer, count)) + retval = -EFAULT; + else + retval = count; + } + + +The usb_bulk_msg function can be very useful for doing single reads or +writes to a device; however, if you need to read or write constantly to +a device, it is recommended to set up your own urbs and submit them to +the USB subsystem. + +When the user program releases the file handle that it has been using to +talk to the device, the release function in the driver is called. In +this function we decrement our private usage count and wait for possible +pending writes: + +:: + + /* decrement our usage count for the device */ + --skel->open_count; + + +One of the more difficult problems that USB drivers must be able to +handle smoothly is the fact that the USB device may be removed from the +system at any point in time, even if a program is currently talking to +it. It needs to be able to shut down any current reads and writes and +notify the user-space programs that the device is no longer there. The +following code (function :c:func:`skel_delete()`) is an example of +how to do this: + +:: + + static inline void skel_delete (struct usb_skel *dev) + { + kfree (dev->bulk_in_buffer); + if (dev->bulk_out_buffer != NULL) + usb_free_coherent (dev->udev, dev->bulk_out_size, + dev->bulk_out_buffer, + dev->write_urb->transfer_dma); + usb_free_urb (dev->write_urb); + kfree (dev); + } + + +If a program currently has an open handle to the device, we reset the +flag ``device_present``. For every read, write, release and other +functions that expect a device to be present, the driver first checks +this flag to see if the device is still present. If not, it releases +that the device has disappeared, and a -ENODEV error is returned to the +user-space program. When the release function is eventually called, it +determines if there is no device and if not, it does the cleanup that +the skel_disconnect function normally does if there are no open files +on the device (see Listing 5). + +Isochronous Data +================ + +This usb-skeleton driver does not have any examples of interrupt or +isochronous data being sent to or from the device. Interrupt data is +sent almost exactly as bulk data is, with a few minor exceptions. +Isochronous data works differently with continuous streams of data being +sent to or from the device. The audio and video camera drivers are very +good examples of drivers that handle isochronous data and will be useful +if you also need to do this. + +Conclusion +========== + +Writing Linux USB device drivers is not a difficult task as the +usb-skeleton driver shows. This driver, combined with the other current +USB drivers, should provide enough examples to help a beginning author +create a working driver in a minimal amount of time. The linux-usb-devel +mailing list archives also contain a lot of helpful information. + +Resources +========= + +The Linux USB Project: +`http://www.linux-usb.org/ `__ + +Linux Hotplug Project: +`http://linux-hotplug.sourceforge.net/ `__ + +Linux USB Working Devices List: +`http://www.qbik.ch/usb/devices/ `__ + +linux-usb-devel Mailing List Archives: +http://marc.theaimsgroup.com/?l=linux-usb-devel + +Programming Guide for Linux USB Device Drivers: +http://usb.cs.tum.edu/usbdoc + +USB Home Page: http://www.usb.org -- 2.9.3