Re: [PATCH 1/3] mfd: apple-ibridge: Add Apple iBridge MFD driver.

[Benjamin Tissoires <benjamin.tissoires@redhat.com>]

On Fri, Apr 26, 2019 at 7:56 AM Life is hard, and then you die
<ronald@innovation.ch> wrote:
>
>
>   Hi Benjamin,
>
> On Thu, Apr 25, 2019 at 11:39:12AM +0200, Benjamin Tissoires wrote:
> > On Thu, Apr 25, 2019 at 10:19 AM Life is hard, and then you die
> > <ronald@innovation.ch> wrote:
> > >
> > >   Hi Benjamin,
> > >
> > > Thank you for looking at this.
> > >
> > > On Wed, Apr 24, 2019 at 04:18:23PM +0200, Benjamin Tissoires wrote:
> > > > On Mon, Apr 22, 2019 at 5:13 AM Ronald Tschalär <ronald@innovation.ch> wrote:
> > > > >
> > > > > The iBridge device provides access to several devices, including:
> > > > > - the Touch Bar
> > > > > - the iSight webcam
> > > > > - the light sensor
> > > > > - the fingerprint sensor
> > > > >
> > > > > This driver provides the core support for managing the iBridge device
> > > > > and the access to the underlying devices. In particular, since the
> > > > > functionality for the touch bar and light sensor is exposed via USB HID
> > > > > interfaces, and the same HID device is used for multiple functions, this
> > > > > driver provides a multiplexing layer that allows multiple HID drivers to
> > > > > be registered for a given HID device. This allows the touch bar and ALS
> > > > > driver to be separated out into their own modules.
> > > >
> > > > Sorry for coming late to the party, but IMO this series is far too
> > > > complex for what you need.
> > > >
> > > > As I read this and the first comment of drivers/mfd/apple-ibridge.c,
> > > > you need to have a HID driver that multiplex 2 other sub drivers
> > > > through one USB communication.
> > > > For that, you are using MFD, platform driver and you own sauce instead
> > > > of creating a bus.
> > >
> > > Basically correct. To be a bit more precise, there are currently two
> > > hid-devices and two drivers (touchbar and als) involved, with
> > > connections as follows (pardon the ugly ascii art):
> > >
> > >   hdev1  ---  tb-drv
> > >            /
> > >           /
> > >          /
> > >   hdev2  ---  als-drv
> > >
> > > i.e. the touchbar driver talks to both hdev's, and hdev2's events
> > > (reports) are processed by both drivers (though each handles different
> > > reports).
> > >
> > > > So, how about we reuse entirely the HID subsystem which already
> > > > provides the capability you need (assuming I am correct above).
> > > > hid-logitech-dj already does the same kind of stuff and you could:
> > > > - create drivers/hid/hid-ibridge.c that handles USB_ID_PRODUCT_IBRIDGE
> > > > - hid-ibridge will then register itself to the hid subsystem with a
> > > > call to hid_hw_start(hdev, HID_CONNECT_HIDRAW) and
> > > > hid_device_io_start(hdev) to enable the events (so you don't create
> > > > useless input nodes for it)
> > > > - then you add your 2 new devices by calling hid_allocate_device() and
> > > > then hid_add_device(). You can even create a new HID group
> > > > APPLE_IBRIDGE and allocate 2 new PIDs for them to distinguish them
> > > > from the actual USB device.
> > > > - then you have 2 brand new HID devices you can create their driver as
> > > > a regular ones.
> > > >
> > > > hid-ibridge.c would just need to behave like any other hid transport
> > > > driver (see logi_dj_ll_driver in drivers/hid/hid-logitech-dj.c) and
> > > > you can get rid of at least the MFD and the platform part of your
> > > > drivers.
> > > >
> > > > Does it makes sense or am I missing something obvious in the middle?
> > >
> > > Yes, I think I understand, and I think this can work. Basically,
> > > instead of demux'ing at the hid-driver level as I am doing now (i.e.
> > > the iBridge hid-driver forwarding calls to the sub-hid-drivers), we
> > > demux at the hid-device level (events forwarded from iBridge hdev to
> > > all "virtual" sub-hdev's, and requests from sub-hdev's forwarded to
> > > the original hdev via an iBridge ll_driver attached to the
> > > sub-hdev's).
> > >
> > > So I would need to create 3 new "virtual" hid-devices (instances) as
> > > follows:
> > >
> > >   hdev1  ---  vhdev1  ---  tb-drv
> > >                         /
> > >           --  vhdev2  --
> > >          /
> > >   hdev2  ---  vhdev3  ---  als-drv
> > >
> > > (vhdev1 is probably not strictly necessary, but makes things more
> > > consistent).
> >
> > Oh, ok.
> >
> > How about the following:
> >
> > hdev1 and hdev2 are merged together in hid-apple-ibridge.c, and then
> > this driver creates 2 virtual hid drivers that are consistent
> >
> > like
> >
> > hdev1---ibridge-drv---vhdev1---tb-drv
> > hdev2--/           \--vhdev2---als-drv
>
> I don't think this will work. The problem is when the sub-drivers need
> to send a report or usb-command: how to they specify which hdev the
> report/command is destined for? While we could store the original hdev
> in each report (the hid_report's device field), that only works for
> hid_hw_request(), but not for things like hid_hw_raw_request() or
> hid_hw_output_report(). Now, currently I don't use the latter two; but
> I do need to send raw usb control messages in the touchbar driver
> (some commands are not proper hid reports), so it definitely breaks
> down there.
>
> Or am I missing something?
>

I'd need to have a deeper look at the protocol, but you can emulate
pure HID devices by having your ibridge handling a translation from
set/get features/input to the usb control messages. Likewise, nothing
prevents you to slightly rewrite the report descriptors you present to
the als and touchbar to have a clear separation with the report ID.

For example, if both hdev1 and hdev2 use a report ID of 0x01, you
could rewrite the report descriptor so that when you receive a report
with an id of 0x01 you send this to hdev1, but you can also translate
0x11 to a report ID 0x01 to hdev2.
Likewise, report ID 0x42 could be a raw USB control message to the USB
under hdev2.

Note that you will have to write 2 report descriptors for your new
devices, but you can take what makes sense from the original ones, and
just add a new collection with a vendor application with with an
opaque meaning (for the USB control messages).

Cheers,
Benjamin