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From:   Jason Andryuk <jandryuk@gmail.com>
Date:   Thu, 20 Oct 2022 08:56:27 -0400
Message-ID: <CAKf6xpvd6rg4AP1XfhCLo0K+BfhVrydBhJy8TReAaTD_1zcY0g@mail.gmail.com>
Subject: Re: [PATCH] Input: xen-kbdfront - drop keys to shrink modalias
To:     Mattijs Korpershoek <mkorpershoek@baylibre.com>
Cc:     linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        Phillip Susi <phill@thesusis.net>, stable@vger.kernel.org,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        linux-input@vger.kernel.org
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On Thu, Oct 20, 2022 at 8:21 AM Jason Andryuk <jandryuk@gmail.com> wrote:
>
> On Thu, Oct 20, 2022 at 4:31 AM Mattijs Korpershoek
> <mkorpershoek@baylibre.com> wrote:
> >
> > On Wed, Oct 19, 2022 at 16:14, Jason Andryuk <jandryuk@gmail.com> wrote:
> >
> > > xen kbdfront registers itself as being able to deliver *any* key since
> > > it doesn't know what keys the backend may produce.
> > >
> > > Unfortunately, the generated modalias gets too large and uevent creation
> > > fails with -ENOMEM.
> > >
> > > This can lead to gdm not using the keyboard since there is no seat
> > > associated [1] and the debian installer crashing [2].
> > >
> > > Trim the ranges of key capabilities by removing some BTN_* ranges.
> > > While doing this, some neighboring undefined ranges are removed to trim
> > > it further.
> > >
> > > This removes:
> > > BTN_DPAD_UP(0x220)..BTN_DPAD_RIGHT(0x223)
> > > Empty space 0x224..0x229
> > >
> > > Emtpy space 0x2bd..0x2bf
> > > BTN_TRIGGER_HAPPY(0x2c0)..BTN_TRIGGER_HAPPY40(0x2e7)
> > > Empty space 0x2e8..0x2ff
> > >
> > > The modalias shrinks from 2082 to 1754 bytes.
> > >
> > > [1] https://github.com/systemd/systemd/issues/22944
> > > [2] https://lore.kernel.org/xen-devel/87o8dw52jc.fsf@vps.thesusis.net/T/
> > >
> > > Cc: Phillip Susi <phill@thesusis.net>
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
> >
> > Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
>
> This patch doesn't work and needs to be withdrawn.  My gdm/udev
> workaround was still active when I tested, so that is why I had a
> working keyboard.  Sorry about that.
>
> Now the question is, which additional keys can be omitted to trim the
> modalias to an acceptable size?
>
> Regards,
> Jason
>
> > > ---
> > >  drivers/input/misc/xen-kbdfront.c | 9 ++++++++-
> > >  1 file changed, 8 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/input/misc/xen-kbdfront.c b/drivers/input/misc/xen-kbdfront.c
> > > index 8d8ebdc2039b..23f37211be78 100644
> > > --- a/drivers/input/misc/xen-kbdfront.c
> > > +++ b/drivers/input/misc/xen-kbdfront.c
> > > @@ -256,7 +256,14 @@ static int xenkbd_probe(struct xenbus_device *dev,
> > >               __set_bit(EV_KEY, kbd->evbit);
> > >               for (i = KEY_ESC; i < KEY_UNKNOWN; i++)
> > >                       __set_bit(i, kbd->keybit);
> > > -             for (i = KEY_OK; i < KEY_MAX; i++)
> > > +             /* In theory we want to go KEY_OK..KEY_MAX, but that grows the
> > > +              * modalias line too long.  KEY_KBD_LCD_MENU5 is the last
> > > +              * defined non-button key. There is a gap of buttons from
> > > +              * BTN_DPAD_UP..BTN_DPAD_RIGHT and KEY_ALS_TOGGLE is the next
> > > +              * defined. */
> > > +             for (i = KEY_OK; i < BTN_DPAD_UP; i++)
> > > +                     __set_bit(i, kbd->keybit);
> > > +             for (i = KEY_ALS_TOGGLE; i <= KEY_KBD_LCD_MENU5; i++)

Changing the upper bound to KEY_BRIGHTNESS_MENU works.  That trims out
KEY_MACRO* and KEY_KBD_LCD_MENU*.

Something has to get trimmed out to bring the size down.  These are
probably less common and okay to remove, but I'm just guessing.

Regards,
Jason