[PATCH 6/7] ALSA: doc: Brush up the old writing-an-alsa-driver

From: Takashi Iwai <tiwai@suse.de>
To: alsa-devel@alsa-project.org
Subject: [PATCH 6/7] ALSA: doc: Brush up the old writing-an-alsa-driver
Date: Thu, 20 Sep 2018 17:55:01 +0200	[thread overview]
Message-ID: <20180920155502.7534-7-tiwai@suse.de> (raw)
In-Reply-To: <20180920155502.7534-1-tiwai@suse.de>

Slightly brushing up and throw the old dust away from my ancient
writing-an-alsa-driver document.  The contents aren't changed so much
but the obsoleted parts are dropped.

Also, remove the date and the version number.  It's useless.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 .../kernel-api/writing-an-alsa-driver.rst     | 307 +++++++++---------
 1 file changed, 149 insertions(+), 158 deletions(-)

diff --git a/Documentation/sound/kernel-api/writing-an-alsa-driver.rst b/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
index a0b268466cb1..b60a26807420 100644
--- a/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
+++ b/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
@@ -3,8 +3,6 @@ Writing an ALSA Driver
 ======================
 
 :Author: Takashi Iwai <tiwai@suse.de>
-:Date:   Oct 15, 2007
-:Edition: 0.3.7
 
 Preface
 =======
@@ -21,11 +19,6 @@ explain the general topic of linux kernel coding and doesn't cover
 low-level driver implementation details. It only describes the standard
 way to write a PCI sound driver on ALSA.
 
-If you are already familiar with the older ALSA ver.0.5.x API, you can
-check the drivers such as ``sound/pci/es1938.c`` or
-``sound/pci/maestro3.c`` which have also almost the same code-base in
-the ALSA 0.5.x tree, so you can compare the differences.
-
 This document is still a draft version. Any feedback and corrections,
 please!!
 
@@ -35,24 +28,7 @@ File Tree Structure
 General
 -------
 
-The ALSA drivers are provided in two ways.
-
-One is the trees provided as a tarball or via cvs from the ALSA's ftp
-site, and another is the 2.6 (or later) Linux kernel tree. To
-synchronize both, the ALSA driver tree is split into two different
-trees: alsa-kernel and alsa-driver. The former contains purely the
-source code for the Linux 2.6 (or later) tree. This tree is designed
-only for compilation on 2.6 or later environment. The latter,
-alsa-driver, contains many subtle files for compiling ALSA drivers
-outside of the Linux kernel tree, wrapper functions for older 2.2 and
-2.4 kernels, to adapt the latest kernel API, and additional drivers
-which are still in development or in tests. The drivers in alsa-driver
-tree will be moved to alsa-kernel (and eventually to the 2.6 kernel
-tree) when they are finished and confirmed to work fine.
-
-The file tree structure of ALSA driver is depicted below. Both
-alsa-kernel and alsa-driver have almost the same file structure, except
-for “core” directory. It's named as “acore” in alsa-driver tree.
+The file tree structure of ALSA driver is depicted below.
 
 ::
 
@@ -61,14 +37,11 @@ for “core” directory. It's named as “acore” in alsa-driver tree.
                             /oss
                             /seq
                                     /oss
-                                    /instr
-                    /ioctl32
                     /include
                     /drivers
                             /mpu401
                             /opl3
                     /i2c
-                            /l3
                     /synth
                             /emux
                     /pci
@@ -80,6 +53,7 @@ for “core” directory. It's named as “acore” in alsa-driver tree.
                     /sparc
                     /usb
                     /pcmcia /(cards)
+                    /soc
                     /oss
 
 
@@ -99,13 +73,6 @@ directory. The rawmidi OSS emulation is included in the ALSA rawmidi
 code since it's quite small. The sequencer code is stored in
 ``core/seq/oss`` directory (see `below <#core-seq-oss>`__).
 
-core/ioctl32
-~~~~~~~~~~~~
-
-This directory contains the 32bit-ioctl wrappers for 64bit architectures
-such like x86-64, ppc64 and sparc64. For 32bit and alpha architectures,
-these are not compiled.
-
 core/seq
 ~~~~~~~~
 
@@ -119,11 +86,6 @@ core/seq/oss
 
 This contains the OSS sequencer emulation codes.
 
-core/seq/instr
-~~~~~~~~~~~~~~
-
-This directory contains the modules for the sequencer instrument layer.
-
 include directory
 -----------------
 
@@ -161,11 +123,6 @@ Although there is a standard i2c layer on Linux, ALSA has its own i2c
 code for some cards, because the soundcard needs only a simple operation
 and the standard i2c API is too complicated for such a purpose.
 
-i2c/l3
-~~~~~~
-
-This is a sub-directory for ARM L3 i2c.
-
 synth directory
 ---------------
 
@@ -209,11 +166,19 @@ The PCMCIA, especially PCCard drivers will go here. CardBus drivers will
 be in the pci directory, because their API is identical to that of
 standard PCI cards.
 
+soc directory
+-------------
+
+This directory contains the codes for ASoC (ALSA System on Chip)
+layer including ASoC core, codec and machine drivers.
+
 oss directory
 -------------
 
-The OSS/Lite source files are stored here in Linux 2.6 (or later) tree.
-In the ALSA driver tarball, this directory is empty, of course :)
+Here contains OSS/Lite codes.
+All codes have been deprecated except for dmasound on m68k as of
+writing this.
+
 
 Basic Flow for PCI Drivers
 ==========================
@@ -352,10 +317,8 @@ to details explained in the following section.
 
               /* (3) */
               err = snd_mychip_create(card, pci, &chip);
-              if (err < 0) {
-                      snd_card_free(card);
-                      return err;
-              }
+              if (err < 0)
+                      goto error;
 
               /* (4) */
               strcpy(card->driver, "My Chip");
@@ -368,22 +331,23 @@ to details explained in the following section.
 
               /* (6) */
               err = snd_card_register(card);
-              if (err < 0) {
-                      snd_card_free(card);
-                      return err;
-              }
+              if (err < 0)
+                      goto error;
 
               /* (7) */
               pci_set_drvdata(pci, card);
               dev++;
               return 0;
+
+      error:
+              snd_card_free(card);
+	      return err;
       }
 
       /* destructor -- see the "Destructor" sub-section */
       static void snd_mychip_remove(struct pci_dev *pci)
       {
               snd_card_free(pci_get_drvdata(pci));
-              pci_set_drvdata(pci, NULL);
       }
 
 
@@ -445,14 +409,26 @@ In this part, the PCI resources are allocated.
   struct mychip *chip;
   ....
   err = snd_mychip_create(card, pci, &chip);
-  if (err < 0) {
-          snd_card_free(card);
-          return err;
-  }
+  if (err < 0)
+          goto error;
 
 The details will be explained in the section `PCI Resource
 Management`_.
 
+When something goes wrong, the probe function needs to deal with the
+error.  In this example, we have a single error handling path placed
+at the end of the function.
+
+::
+
+  error:
+          snd_card_free(card);
+	  return err;
+
+Since each component can be properly freed, the single
+:c:func:`snd_card_free()` call should suffice in most cases.
+
+
 4) Set the driver ID and name strings.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -486,10 +462,8 @@ too.
 ::
 
   err = snd_card_register(card);
-  if (err < 0) {
-          snd_card_free(card);
-          return err;
-  }
+  if (err < 0)
+          goto error;
 
 Will be explained in the section `Management of Cards and
 Components`_, too.
@@ -513,14 +487,13 @@ The destructor, remove callback, simply releases the card instance. Then
 the ALSA middle layer will release all the attached components
 automatically.
 
-It would be typically like the following:
+It would be typically just :c:func:`calling snd_card_free()`:
 
 ::
 
   static void snd_mychip_remove(struct pci_dev *pci)
   {
           snd_card_free(pci_get_drvdata(pci));
-          pci_set_drvdata(pci, NULL);
   }
 
 
@@ -546,7 +519,7 @@ in the source file. If the code is split into several files, the files
 without module options don't need them.
 
 In addition to these headers, you'll need ``<linux/interrupt.h>`` for
-interrupt handling, and ``<asm/io.h>`` for I/O access. If you use the
+interrupt handling, and ``<linux/io.h>`` for I/O access. If you use the
 :c:func:`mdelay()` or :c:func:`udelay()` functions, you'll need
 to include ``<linux/delay.h>`` too.
 
@@ -720,6 +693,13 @@ function, which will call the real destructor.
 
 where :c:func:`snd_mychip_free()` is the real destructor.
 
+The demerit of this method is the obviously more amount of codes.
+The merit is, however, you can trigger the own callback at registering
+and disconnecting the card via setting in snd_device_ops.
+About the registering and disconnecting the card, see the subsections
+below.
+
+
 Registration and Release
 ------------------------
 
@@ -905,10 +885,8 @@ Resource Allocation
 -------------------
 
 The allocation of I/O ports and irqs is done via standard kernel
-functions. Unlike ALSA ver.0.5.x., there are no helpers for that. And
-these resources must be released in the destructor function (see below).
-Also, on ALSA 0.9.x, you don't need to allocate (pseudo-)DMA for PCI
-like in ALSA 0.5.x.
+functions.  These resources must be released in the destructor
+function (see below).
 
 Now assume that the PCI device has an I/O port with 8 bytes and an
 interrupt. Then :c:type:`struct mychip <mychip>` will have the
@@ -1064,7 +1042,8 @@ and the allocation would be like below:
 
 ::
 
-  if ((err = pci_request_regions(pci, "My Chip")) < 0) {
+  err = pci_request_regions(pci, "My Chip");
+  if (err < 0) {
           kfree(chip);
           return err;
   }
@@ -1086,6 +1065,21 @@ and the corresponding destructor would be:
           ....
   }
 
+Of course, a modern way with :c:func:`pci_iomap()` will make things a
+bit easier, too.
+
+::
+
+  err = pci_request_regions(pci, "My Chip");
+  if (err < 0) {
+          kfree(chip);
+          return err;
+  }
+  chip->iobase_virt = pci_iomap(pci, 0, 0);
+
+which is paired with :c:func:`pci_iounmap()` at destructor.
+
+
 PCI Entries
 -----------
 
@@ -1154,13 +1148,6 @@ And at last, the module entries:
 Note that these module entries are tagged with ``__init`` and ``__exit``
 prefixes.
 
-Oh, one thing was forgotten. If you have no exported symbols, you need
-to declare it in 2.2 or 2.4 kernels (it's not necessary in 2.6 kernels).
-
-::
-
-  EXPORT_NO_SYMBOLS;
-
 That's all!
 
 PCM Interface
@@ -2113,6 +2100,16 @@ non-contiguous buffers. The mmap calls this callback to get the page
 address. Some examples will be explained in the later section `Buffer
 and Memory Management`_, too.
 
+mmap calllback
+~~~~~~~~~~~~~~
+
+This is another optional callback for controlling mmap behavior.
+Once when defined, PCM core calls this callback when a page is
+memory-mapped instead of dealing via the standard helper.
+If you need special handling (due to some architecture or
+device-specific issues), implement everything here as you like.
+
+
 PCM Interrupt Handler
 ---------------------
 
@@ -2370,6 +2367,27 @@ to define the inverse rule:
                       hw_rule_format_by_channels, NULL,
                       SNDRV_PCM_HW_PARAM_CHANNELS, -1);
 
+One typical usage of the hw constraints is to align the buffer size
+with the period size.  As default, ALSA PCM core doesn't enforce the
+buffer size to be aligned with the period size.  For example, it'd be
+possible to have a combination like 256 period bytes with 999 buffer
+bytes.
+
+Many device chips, however, require the buffer to be a multiple of
+periods.  In such a case, call
+:c:func:`snd_pcm_hw_constraint_integer()` for
+``SNDRV_PCM_HW_PARAM_PERIODS``.
+
+::
+
+  snd_pcm_hw_constraint_integer(substream->runtime,
+                                SNDRV_PCM_HW_PARAM_PERIODS);
+
+This assures that the number of periods is integer, hence the buffer
+size is aligned with the period size.
+
+The hw constraint is a very much powerful mechanism to define the
+preferred PCM configuration, and there are relevant helpers.
 I won't give more details here, rather I would like to say, “Luke, use
 the source.”
 
@@ -3712,7 +3730,14 @@ example, for an intermediate buffer. Since the allocated pages are not
 contiguous, you need to set the ``page`` callback to obtain the physical
 address at every offset.
 
-The implementation of ``page`` callback would be like this:
+The easiest way to achieve it would be to use
+:c:func:`snd_pcm_lib_alloc_vmalloc_buffer()` for allocating the buffer
+via :c:func:`vmalloc()`, and set :c:func:`snd_pcm_sgbuf_ops_page()` to
+the ``page`` callback.  At release, you need to call
+:c:func:`snd_pcm_lib_free_vmalloc_buffer()`.
+
+If you want to implementation the ``page`` manually, it would be like
+this:
 
 ::
 
@@ -3848,7 +3873,9 @@ Power Management
 
 If the chip is supposed to work with suspend/resume functions, you need
 to add power-management code to the driver. The additional code for
-power-management should be ifdef-ed with ``CONFIG_PM``.
+power-management should be ifdef-ed with ``CONFIG_PM``, or annotated
+with __maybe_unused attribute; otherwise the compiler will complain
+you.
 
 If the driver *fully* supports suspend/resume that is, the device can be
 properly resumed to its state when suspend was called, you can set the
@@ -3879,18 +3906,16 @@ the case of PCI drivers, the callbacks look like below:
 
 ::
 
-  #ifdef CONFIG_PM
-  static int snd_my_suspend(struct pci_dev *pci, pm_message_t state)
+  static int __maybe_unused snd_my_suspend(struct device *dev)
   {
           .... /* do things for suspend */
           return 0;
   }
-  static int snd_my_resume(struct pci_dev *pci)
+  static int __maybe_unused snd_my_resume(struct device *dev)
   {
           .... /* do things for suspend */
           return 0;
   }
-  #endif
 
 The scheme of the real suspend job is as follows.
 
@@ -3909,18 +3934,14 @@ The scheme of the real suspend job is as follows.
 
 6. Stop the hardware if necessary.
 
-7. Disable the PCI device by calling
-   :c:func:`pci_disable_device()`. Then, call
-   :c:func:`pci_save_state()` at last.
-
 A typical code would be like:
 
 ::
 
-  static int mychip_suspend(struct pci_dev *pci, pm_message_t state)
+  static int __maybe_unused mychip_suspend(struct device *dev)
   {
           /* (1) */
-          struct snd_card *card = pci_get_drvdata(pci);
+          struct snd_card *card = dev_get_drvdata(dev);
           struct mychip *chip = card->private_data;
           /* (2) */
           snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
@@ -3932,9 +3953,6 @@ A typical code would be like:
           snd_mychip_save_registers(chip);
           /* (6) */
           snd_mychip_stop_hardware(chip);
-          /* (7) */
-          pci_disable_device(pci);
-          pci_save_state(pci);
           return 0;
   }
 
@@ -3943,44 +3961,35 @@ The scheme of the real resume job is as follows.
 
 1. Retrieve the card and the chip data.
 
-2. Set up PCI. First, call :c:func:`pci_restore_state()`. Then
-   enable the pci device again by calling
-   :c:func:`pci_enable_device()`. Call
-   :c:func:`pci_set_master()` if necessary, too.
+2. Re-initialize the chip.
 
-3. Re-initialize the chip.
+3. Restore the saved registers if necessary.
 
-4. Restore the saved registers if necessary.
+4. Resume the mixer, e.g. calling :c:func:`snd_ac97_resume()`.
 
-5. Resume the mixer, e.g. calling :c:func:`snd_ac97_resume()`.
+5. Restart the hardware (if any).
 
-6. Restart the hardware (if any).
-
-7. Call :c:func:`snd_power_change_state()` with
+6. Call :c:func:`snd_power_change_state()` with
    ``SNDRV_CTL_POWER_D0`` to notify the processes.
 
 A typical code would be like:
 
 ::
 
-  static int mychip_resume(struct pci_dev *pci)
+  static int __maybe_unused mychip_resume(struct pci_dev *pci)
   {
           /* (1) */
-          struct snd_card *card = pci_get_drvdata(pci);
+          struct snd_card *card = dev_get_drvdata(dev);
           struct mychip *chip = card->private_data;
           /* (2) */
-          pci_restore_state(pci);
-          pci_enable_device(pci);
-          pci_set_master(pci);
-          /* (3) */
           snd_mychip_reinit_chip(chip);
-          /* (4) */
+          /* (3) */
           snd_mychip_restore_registers(chip);
-          /* (5) */
+          /* (4) */
           snd_ac97_resume(chip->ac97);
-          /* (6) */
+          /* (5) */
           snd_mychip_restart_chip(chip);
-          /* (7) */
+          /* (6) */
           snd_power_change_state(card, SNDRV_CTL_POWER_D0);
           return 0;
   }
@@ -4046,15 +4055,14 @@ And next, set suspend/resume callbacks to the pci_driver.
 
 ::
 
+  static SIMPLE_DEV_PM_OPS(snd_my_pm_ops, mychip_suspend, mychip_resume);
+
   static struct pci_driver driver = {
           .name = KBUILD_MODNAME,
           .id_table = snd_my_ids,
           .probe = snd_my_probe,
           .remove = snd_my_remove,
-  #ifdef CONFIG_PM
-          .suspend = snd_my_suspend,
-          .resume = snd_my_resume,
-  #endif
+          .driver.pm = &snd_my_pm_ops,
   };
 
 Module Parameters
@@ -4078,7 +4086,7 @@ variables, instead. ``enable`` option is not always necessary in this
 case, but it would be better to have a dummy option for compatibility.
 
 The module parameters must be declared with the standard
-``module_param()()``, ``module_param_array()()`` and
+``module_param()``, ``module_param_array()`` and
 :c:func:`MODULE_PARM_DESC()` macros.
 
 The typical coding would be like below:
@@ -4094,15 +4102,14 @@ The typical coding would be like below:
   module_param_array(enable, bool, NULL, 0444);
   MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
 
-Also, don't forget to define the module description, classes, license
-and devices. Especially, the recent modprobe requires to define the
+Also, don't forget to define the module description and the license.
+Especially, the recent modprobe requires to define the
 module license as GPL, etc., otherwise the system is shown as “tainted”.
 
 ::
 
-  MODULE_DESCRIPTION("My Chip");
+  MODULE_DESCRIPTION("Sound driver for My Chip");
   MODULE_LICENSE("GPL");
-  MODULE_SUPPORTED_DEVICE("{{Vendor,My Chip Name}}");
 
 
 How To Put Your Driver Into ALSA Tree
@@ -4117,21 +4124,17 @@ a question now: how to put my own driver into the ALSA driver tree? Here
 
 Suppose that you create a new PCI driver for the card “xyz”. The card
 module name would be snd-xyz. The new driver is usually put into the
-alsa-driver tree, ``alsa-driver/pci`` directory in the case of PCI
-cards. Then the driver is evaluated, audited and tested by developers
-and users. After a certain time, the driver will go to the alsa-kernel
-tree (to the corresponding directory, such as ``alsa-kernel/pci``) and
-eventually will be integrated into the Linux 2.6 tree (the directory
-would be ``linux/sound/pci``).
+alsa-driver tree, ``sound/pci`` directory in the case of PCI
+cards.
 
 In the following sections, the driver code is supposed to be put into
-alsa-driver tree. The two cases are covered: a driver consisting of a
+Linux kernel tree. The two cases are covered: a driver consisting of a
 single source file and one consisting of several source files.
 
 Driver with A Single Source File
 --------------------------------
 
-1. Modify alsa-driver/pci/Makefile
+1. Modify sound/pci/Makefile
 
    Suppose you have a file xyz.c. Add the following two lines
 
@@ -4160,52 +4163,43 @@ Driver with A Single Source File
 
    For the details of Kconfig script, refer to the kbuild documentation.
 
-3. Run cvscompile script to re-generate the configure script and build
-   the whole stuff again.
-
 Drivers with Several Source Files
 ---------------------------------
 
 Suppose that the driver snd-xyz have several source files. They are
-located in the new subdirectory, pci/xyz.
+located in the new subdirectory, sound/pci/xyz.
 
-1. Add a new directory (``xyz``) in ``alsa-driver/pci/Makefile`` as
-   below
+1. Add a new directory (``sound/pci/xyz``) in ``sound/pci/Makefile``
+   as below
 
 ::
 
-  obj-$(CONFIG_SND) += xyz/
+  obj-$(CONFIG_SND) += sound/pci/xyz/
 
 
-2. Under the directory ``xyz``, create a Makefile
+2. Under the directory ``sound/pci/xyz``, create a Makefile
 
 ::
 
-         ifndef SND_TOPDIR
-         SND_TOPDIR=../..
-         endif
-
-         include $(SND_TOPDIR)/toplevel.config
-         include $(SND_TOPDIR)/Makefile.conf
-
          snd-xyz-objs := xyz.o abc.o def.o
-
          obj-$(CONFIG_SND_XYZ) += snd-xyz.o
 
-         include $(SND_TOPDIR)/Rules.make
-
 3. Create the Kconfig entry
 
    This procedure is as same as in the last section.
 
-4. Run cvscompile script to re-generate the configure script and build
-   the whole stuff again.
 
 Useful Functions
 ================
 
 :c:func:`snd_printk()` and friends
----------------------------------------
+----------------------------------
+
+.. note:: This subsection describes a few helper functions for
+decorating a bit more on the standard :c:func:`printk()` & co.
+However, in general, the use of such helpers is no longer recommended.
+If possible, try to stick with the standard functions like
+:c:func:`dev_err()` or :c:func:`pr_err()`.
 
 ALSA provides a verbose version of the :c:func:`printk()` function.
 If a kernel config ``CONFIG_SND_VERBOSE_PRINTK`` is set, this function
@@ -4221,13 +4215,10 @@ just like :c:func:`snd_printk()`. If the ALSA is compiled without
 the debugging flag, it's ignored.
 
 :c:func:`snd_printdd()` is compiled in only when
-``CONFIG_SND_DEBUG_VERBOSE`` is set. Please note that
-``CONFIG_SND_DEBUG_VERBOSE`` is not set as default even if you configure
-the alsa-driver with ``--with-debug=full`` option. You need to give
-explicitly ``--with-debug=detect`` option instead.
+``CONFIG_SND_DEBUG_VERBOSE`` is set.
 
 :c:func:`snd_BUG()`
-------------------------
+-------------------
 
 It shows the ``BUG?`` message and stack trace as well as
 :c:func:`snd_BUG_ON()` at the point. It's useful to show that a
@@ -4236,7 +4227,7 @@ fatal error happens there.
 When no debug flag is set, this macro is ignored.
 
 :c:func:`snd_BUG_ON()`
-----------------------------
+----------------------
 
 :c:func:`snd_BUG_ON()` macro is similar with
 :c:func:`WARN_ON()` macro. For example, snd_BUG_ON(!pointer); or
-- 
2.18.0

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