linux-kernel - Re: [PATCH v1 1/2] HID: generic: Add ->match() check to __check_hid

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Date:   Wed, 7 Dec 2022 10:54:25 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Jiri Kosina <jikos@...nel.org>,
        Bastien Nocera <hadess@...ess.net>,
        Filipe Laíns <lains@...eup.net>,
        linux-input@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Thorsten Leemhuis <regressions@...mhuis.info>
Subject: Re: [PATCH v1 1/2] HID: generic: Add ->match() check to __check_hid_generic()

On Wed, Dec 7, 2022 at 10:27 AM Benjamin Tissoires
<benjamin.tissoires@...hat.com> wrote:
>
> On Wed, Dec 7, 2022 at 10:13 AM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > Some special HID drivers (for example, hid-logitech-hidpp) use ->match()
> > callbacks to reject specific devices that otherwise would match the
> > driver's device ID list, with the expectation that those devices will
> > be handled by some other drivers.  However, this doesn't work if
> > hid-generic is expected to bind to the given device, because its
> > ->match() callback, hid_generic_match(), rejects all devices that match
> > device ID lists of the other HID drivers regardless of what is returned
> > by the other drivers' ->match() callbacks.
>
> Thanks Rafael for spotting that corner case in the ->match() processing.
>
> >
> > To make it work, amend the function used by hid_generic_match() for
> > checking an individual driver, __check_hid_generic(), with a check
> > involving the given driver's ->match() callback, so 0 is returned
> > when that callback rejects the device in question.
>
> Shouldn't we add that logic to hid_match_device() directly in
> hid-core.c instead?
> It feels wrong to have a function named "hid_match_device()" and have
> to manually call later "->match()" on the driver itself.

Well, I've followed the pattern present in hid_device_probe(), where
hid_match_device() is first called to check against the device ID list
and then ->match() is invoked later only if that doesn't fail.

Also changing hid_match_device() would change the way in which
hid_bus_match() works and that may lead to subsequent regressions,
potentially, so I'd rather avoid doing that ATM.

> Ack on the general idea anyway.

Thanks!

> >
> > Fixes: 532223c8ac57 ("HID: logitech-hidpp: Enable HID++ for all the Logitech Bluetooth devices")
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >  drivers/hid/hid-generic.c |    8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > Index: linux-pm/drivers/hid/hid-generic.c
> > ===================================================================
> > --- linux-pm.orig/drivers/hid/hid-generic.c
> > +++ linux-pm/drivers/hid/hid-generic.c
> > @@ -31,7 +31,13 @@ static int __check_hid_generic(struct de
> >         if (hdrv == &hid_generic)
> >                 return 0;
> >
> > -       return hid_match_device(hdev, hdrv) != NULL;
> > +       if (!hid_match_device(hdev, hdrv))
> > +               return 0;
> > +
> > +       if (hdrv->match)
> > +               return hdrv->match(hdev, false);
> > +
> > +       return 1;
> >  }
> >
> >  static bool hid_generic_match(struct hid_device *hdev,
> >
> >
> >
>