linux-kernel - Re: [PATCH] HID: input: fix a4tech horizontal wheel custom usage id

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Message-ID: <5214eeef4c307c84d8510661af2d9fa2d4e527f3.camel@suse.de>
Date:   Tue, 11 Jun 2019 13:33:11 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] HID: input: fix a4tech horizontal wheel custom usage id

On Tue, 2019-06-11 at 10:43 +0200, Benjamin Tissoires wrote:
> Hi Nicolas,
> 
> On Mon, Jun 10, 2019 at 8:54 PM Nicolas Saenz Julienne
> <nsaenzjulienne@...e.de> wrote:
> > Some a4tech mice use the 'GenericDesktop.00b8' usage id to inform
> > whether the previous wheel report was horizontal or vertical. Before
> > c01908a14bf73 ("HID: input: add mapping for "Toggle Display" key") this
> > usage id was being mapped to 'Relative.Misc'. After the patch it's
> > simply ignored (usage->type == 0 & usage->code == 0). Checking the HID
> > Usage Tables it turns out it's a reserved usage_id, so it makes sense to
> > map it the way it was. Ultimately this makes hid-a4tech ignore the
> > WHEEL/HWHEEL selection event, as it has no usage->type.
> > 
> > The patch reverts the handling of the usage id back to it's previous
> > behavior.
> 
> Hmm, if A4Tech is using a reserved usage, we shouldn't fix this in
> hid-input.c but in hid-a4tech instead.
> Because you won't know when someone else in the HID consortium will
> remap this usage to some other random axis, and your mouse will be
> broken again.
> 
> How about you add a .input_mapping callback in hid-a4tech and map this
> usage there to your needs? This way you will be sure that such a
> situation will not happen again.

I agree it would be a cleaner solution.

In summary the first report indicates the wheel relative value, the second the
orientation. The first report is already being mapped to REL_WHEEL and
REL_WHEEL (or the high-res versions), but what would be a correct code for the
second report? The way I see it, we shouldn't map it to anything. And then
catch both events in the custom driver to build the input_event() accordinlgy
(as it's almost being done already). Is this somewhat correct? I'll send a
followup patch anyway so we have something more tangible comment on.

> > Fixes: c01908a14bf73 ("HID: input: add mapping for "Toggle Display" key")
> > Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
> > ---
> >  drivers/hid/hid-input.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
> > index 63855f275a38..6a956d5a195e 100644
> > --- a/drivers/hid/hid-input.c
> > +++ b/drivers/hid/hid-input.c
> > @@ -671,7 +671,7 @@ static void hidinput_configure_usage(struct hid_input
> > *hidinput, struct hid_fiel
> >                 if ((usage->hid & 0xf0) == 0xb0) {      /* SC - Display */
> >                         switch (usage->hid & 0xf) {
> >                         case 0x05: map_key_clear(KEY_SWITCHVIDEOMODE);
> > break;
> > -                       default: goto ignore;
> > +                       default: goto unknown;
> >                         }
> >                         break;
> >                 }
> > --
> > 2.21.0
> > 


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