linux-kernel - Re: [PATCH] Input: reserve 2 events code because of HID

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Message-ID: <CAO-hwJLpf0-aFRG1yhfSxJA20bw5_HTH+LcwQ9+ys4UE1YVNRw@mail.gmail.com>
Date:   Thu, 4 Oct 2018 14:22:33 +0200
From:   Benjamin Tissoires <benjamin.tissoires@...hat.com>
To:     Peter Hutterer <peter.hutterer@...-t.net>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Jiri Kosina <jikos@...nel.org>, hcutts@...omium.org,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Input: reserve 2 events code because of HID

Oops, looks like this one fell through the cracks.

On Sat, Sep 8, 2018 at 3:44 AM Peter Hutterer <peter.hutterer@...-t.net> wrote:
>
> On Fri, Sep 07, 2018 at 10:51:15AM +0200, Benjamin Tissoires wrote:
> > From: Benjamin Tissoires <benjamin.tissoires@...hat.com>
> >
> > Prior to commit 190d7f02ce8e ("HID: input: do not increment usages when
> > a duplicate is found") from the v4.18 kernel, HID used to shift the
> > event codes if a duplicate usage was found. This ended up in a situation
> > where a device would export a ton of ABS_MISC+n event codes, or a ton
> > of REL_MISC+n event codes.
> >
> > This is now fixed, however userspace needs to detect those situation.
> > Fortunately, ABS_MISC+1 was never assigned a code, and so libinput
> > can detect fake multitouch devices from genuine ones by checking is
> > ABS_MISC+1 is set.
>
> sorry, this isn't quite correct. we use ABS_MT_SLOT - 1 (0x2e) for the
> detection of fake MT devices, i.e.
>   if (ABS_MT_SLOT and not ABS_MT_SLOT-1) then multitouch

Will send a v2 ASAP.

>
> That gives you up to ABS_MISC + 6 for legitimate usage. this is handled by
> libevdev, not libinput directly. libevdev adjusts the various bits on "is
> this device an MT device" based on whether ABS_MT_SLOT-1 is set.
>
> I can change this to also check for (ABS_MISC and not ABS_MISC+1) but that
> obviously will depend on updated libraries then. Though I guess it won't
> really be an issue until we fill up the other codes up to including 0x2e
> with real values and expect userspace to handle those.

Nah, better changing the value here.

>
> None of the bits I maintain have special code for REL_MISC+n so that bit
> works fine, IMO.
>
> One request though: instead of just having the value reserved, can we make
> it REL_RESERVED and ABS_RESERVED please? Or ABS_CANARY :) Much easier than
> hardcoding the numeric value.

My first thoughts were that ABS_CANARY has the inconvenient of being
too tempting to be used as a real value.
OTOH, REL_RESERVED and ABS_RESERVED should be fine.

I'll send out a v2 with those changes.

Cheers,
Benjamin

>
> Cheers,
>    Peter
>
>
> > Now that we have REL_WHEEL_HI_RES, libinput won't be able to differentiate
> > true high res mice from some other device in a pre-v4.18 kernel.
> >
> > Set in stone that the ABS_MISC+1 and REL_MISC+1 are reserved and should not
> > be used so userspace can properly work around those old kernels.
> >
> > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>
> > ---
> >
> > Hi,
> >
> > while reviewing my local tree, I realize that we might want to be able
> > to differentiate older kernels from new ones that export REL_WHEEL_HI_RES.
> >
> > I know Dmitry was against adding several REL_MISC, so I hope just moving
> > REL_WHEEL_HI_RES by one and reserving the faulty event codes would be good
> > this time.
> >
> > This patch applies on top of the branch for-4.20/logitech-highres from
> > Jiri's tree. It should go through Jiri's tree as well.
> >
> > Cheers,
> > Benjamin
> >
> >  include/uapi/linux/input-event-codes.h | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/uapi/linux/input-event-codes.h b/include/uapi/linux/input-event-codes.h
> > index 29fb891ea337..30149939249a 100644
> > --- a/include/uapi/linux/input-event-codes.h
> > +++ b/include/uapi/linux/input-event-codes.h
> > @@ -708,7 +708,12 @@
> >  #define REL_DIAL             0x07
> >  #define REL_WHEEL            0x08
> >  #define REL_MISC             0x09
> > -#define REL_WHEEL_HI_RES     0x0a
> > +/*
> > + * 0x0a is reserved and should not be used.
> > + * It was used by HID as REL_MISC+1 and usersapce needs to detect if
> > + * the next REL_* event is correct or is just REL_MISC + n.
> > + */
> > +#define REL_WHEEL_HI_RES     0x0b
> >  #define REL_MAX                      0x0f
> >  #define REL_CNT                      (REL_MAX+1)
> >
> > @@ -745,6 +750,12 @@
> >
> >  #define ABS_MISC             0x28
> >
> > +/*
> > + * 0x29 is reserved and should not be used.
> > + * It was used by HID as ABS_MISC+1 and usersapce needs to detect if
> > + * the next ABS_* event is correct or is just ABS_MISC + n.
> > + */
> > +
> >  #define ABS_MT_SLOT          0x2f    /* MT slot being modified */
> >  #define ABS_MT_TOUCH_MAJOR   0x30    /* Major axis of touching ellipse */
> >  #define ABS_MT_TOUCH_MINOR   0x31    /* Minor axis (omit if circular) */
> > --
> > 2.14.3
> >