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Date:   Thu, 14 Jan 2021 10:43:45 -0800
From:   Dmitry Torokhov <dmitry.torokhov@gmail.com>
To:     Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc:     Jiri Kosina <jikos@kernel.org>,
        Peter Hutterer <peter.hutterer@who-t.net>,
        Sean O'Brien <seobrien@chromium.org>,
        "open list:HID CORE LAYER" <linux-input@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] HID: hid-input: avoid splitting keyboard, system and
 consumer controls
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Hi Benjamin,

On Thu, Jan 14, 2021 at 10:23:02AM +0100, Benjamin Tissoires wrote:
> Hi Dmitry,
> 
> On Thu, Jan 14, 2021 at 7:24 AM Dmitry Torokhov
> <dmitry.torokhov@gmail.com> wrote:
> >
> > A typical USB keyboard usually splits its keys into several reports:
> >
> > - one for the basic alphanumeric keys, modifier keys, F<n> keys, six pack
> >   keys and keypad. This report's application is normally listed as
> >   GenericDesktop.Keyboard
> > - a GenericDesktop.SystemControl report for the system control keys, such
> >   as power and sleep
> > - Consumer.ConsumerControl report for multimedia (forward, rewind,
> >   play/pause, mute, etc) and other extended keys.
> > - additional output, vendor specific, and feature reports
> >
> > Splitting each report into a separate input device is wasteful and even
> > hurts userspace as it makes it harder to determine the true capabilities
> > (set of available keys) of a keyboard, so let's adjust application
> > matching to merge system control and consumer control reports with
> > keyboard report, if one has already been processed.
> >
> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > ---
> >  drivers/hid/hid-input.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
> > index f797659cb9d9..df45d8d07dc2 100644
> > --- a/drivers/hid/hid-input.c
> > +++ b/drivers/hid/hid-input.c
> > @@ -1851,6 +1851,16 @@ static struct hid_input *hidinput_match_application(struct hid_report *report)
> >         list_for_each_entry(hidinput, &hid->inputs, list) {
> >                 if (hidinput->application == report->application)
> >                         return hidinput;
> > +
> > +               /*
> > +                * Keep SystemControl and ConsumerControl applications together
> > +                * with the main keyboard, if present.
> > +                */
> > +               if ((report->application == HID_GD_SYSTEM_CONTROL ||
> > +                    report->application == HID_CP_CONSUMER_CONTROL) &&
> > +                   hidinput->application == HID_GD_KEYBOARD) {
> 
> I am not fundamentally against the patch, but I think that if the
> device exposes first a HID_CP_CONSUMER_CONTROL and then a
> HID_GD_KEYBOARD we will end up with 2 different input nodes. We likely
> need to "convert" HID_GD_SYSTEM_CONTROL and HID_CP_CONSUMER_CONTROL to
> HID_GD_KEYBOARD when creating the hidinput.

While it is technically possible that consumer control or system control
comes first, before main keyboard report, in reality all keyboards that
I have seen so far have the main keyboard report first, so I opted not
to handle the uncommon case to keep the code simple.

I could add the above as a comment, and we could wait to see if there
are devices that are exceptions to the common practice, or I can go and
try to implement the conversion if you feel it is required. Please let
me know.

Note that we will still end up with 2+ input nodes if a device uses
several USB interfaces, but we can't really do much about such cases
(well, short of having a specialized driver claiming additional
interfaces while probing the first one, but that is really outside of
scope of this patch).

Thanks!

-- 
Dmitry