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Date:   Thu, 25 Apr 2019 22:56:32 -0700
From:   "Life is hard, and then you die" <ronald@innovation.ch>
To:     Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc:     Jiri Kosina <jikos@kernel.org>,
        Jonathan Cameron <jic23@kernel.org>,
        Hartmut Knaack <knaack.h@gmx.de>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
        Lee Jones <lee.jones@linaro.org>,
        "open list:HID CORE LAYER" <linux-input@vger.kernel.org>,
        linux-iio@vger.kernel.org, lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/3] mfd: apple-ibridge: Add Apple iBridge MFD driver.
Message-ID: <20190426055632.GC31266@innovation.ch>
References: <20190422031251.11968-1-ronald@innovation.ch>
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  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 Tschal?r <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?


  Cheers,

  Ronald