linux-kernel - Re: [PATCH v2] HID: google: modify HID device groups of eel

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Date:   Fri, 14 Jan 2022 11:59:50 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Zhengqiao Xia <xiazhengqiao@...qin.corp-partner.google.com>,
        benjamin.tissoires@...hat.com, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org, dianders@...omium.org,
        Wei-Ning Huang <wnhuang@...gle.com>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Sean O'Brien <seobrien@...omium.org>, phoenixshen@...omium.org
Subject: Re: [PATCH v2] HID: google: modify HID device groups of eel

On Fri, Jan 14, 2022 at 01:25:12PM -0600, Stephen Boyd wrote:
> Quoting Jiri Kosina (2022-01-14 00:38:23)
> > On Fri, 7 Jan 2022, Zhengqiao Xia wrote:
> >
> > > If HID_GROUP of eel is set to HID_GROUP_GENERIC, Whiskers Tablet
> > > Mode Switch of eel hammer will not be detected by system. when it
> > > is set to HID_GROUP_VIVALDI, system will detect Whiskers Tablet
> > > Mode Switch successfully.
> > >
> > > Signed-off-by: Zhengqiao Xia <xiazhengqiao@...qin.corp-partner.google.com>
> > > ---
> > >  drivers/hid/hid-google-hammer.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/hid/hid-google-hammer.c b/drivers/hid/hid-google-hammer.c
> > > index 0403beb3104b..e5acd15f4a55 100644
> > > --- a/drivers/hid/hid-google-hammer.c
> > > +++ b/drivers/hid/hid-google-hammer.c
> > > @@ -585,7 +585,7 @@ static void hammer_remove(struct hid_device *hdev)
> > >  static const struct hid_device_id hammer_devices[] = {
> > >       { HID_DEVICE(BUS_USB, HID_GROUP_GENERIC,
> > >                    USB_VENDOR_ID_GOOGLE, USB_DEVICE_ID_GOOGLE_DON) },
> > > -     { HID_DEVICE(BUS_USB, HID_GROUP_GENERIC,
> > > +     { HID_DEVICE(BUS_USB, HID_GROUP_VIVALDI,
> > >                    USB_VENDOR_ID_GOOGLE, USB_DEVICE_ID_GOOGLE_EEL) },
> > >       { HID_DEVICE(BUS_USB, HID_GROUP_GENERIC,
> >
> > Color me confused, but anything with HID_GROUP_VIVALDI should be matched
> > by hid-vivaldi driver, so what is this about?

We need this particular chunk because hid_scan_collection() forces all
devices that declare usage 0x01 from the Google Vendor page to be marked
as HID_GROUP_VIVALDI, so without it the hammer driver will not match
eel.

However this change (I believe) will break vivaldi functionality
(reporting of the top row map) so we can't apply this as is.

> >
> 
> My understanding is that 'vivaldi' is mostly a keyboard layout and
> 'hammer' is a detachable keyboard. We want to prevent the hid-vivaldi
> driver from probing this particular device because the hid-vivaldi
> driver doesn't know about detachable keyboards. Hammer devices also
> support 360 degree wraparound so we know that the keyboard has been put
> behind the screen or that it's being used to stand up the device on a
> table.
> 
> Given all that, I'm still confused. If we make the hid-google-hammer
> driver probe this device and the keyboard layout is vivaldi then we'd
> want the part of the vivaldi driver that exposes the
> function_row_physmap through sysfs. Otherwise userspace won't know how
> to handle the function row properly. I think we need the device to stack
> two drivers here. Does that happen with HID?

As far as I know HID does not easily allow "stacking" drivers like that.

Probably the easiest way would be to export vivaldi_feature_mapping()
and the show method for the physical row map and call them from the
hammer driver.

Thanks.

-- 
Dmitry