Re: [PATCH] Handle spurious backslash key repeats on some keyboards

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Wed, Sep 10, 2014 at 04:22:13PM +0200, David Herrmann wrote:
> Hi
> 
> On Wed, Sep 10, 2014 at 1:13 PM, Fredrik Hallenberg
> <megahallon@gmail.com> wrote:
> > The keyboards in question have n-key rollover, this seems to be
> > implemented by sending every single key id every time the keyboard is
> > polled, this includes several keys that does not exist physically for
> > example 0x31. This may be a bit excessive but not broken. If I press
> > a-key and '-key in rapid succession, I get the following events
> > (omitting all other keys):
> >
> > VALUE 1 CODE 30 HID 0x4
> > VALUE 0 CODE 43 HID 0x31
> > VALUE 0 CODE 43 HID 0x32
> > Output: a
> >
> > VALUE 1 CODE 30 HID 0x4
> > VALUE 0 CODE 43 HID 0x31  - 43 is up
> > VALUE 1 CODE 43 HID 0x32  - 43 is down
> > Output: '
> >
> > VALUE 0 CODE 30 HID 0x4
> > VALUE 0 CODE 43 HID 0x31  - 43 is up
> > VALUE 1 CODE 43 HID 0x32  - 43 is down
> > Output: '
> >
> > VALUE 0 CODE 30 HID 0x4
> > VALUE 0 CODE 43 HID 0x31  - 43 is up
> > VALUE 1 CODE 43 HID 0x32  - 43 is down
> > Output: '
> >
> > So even though the keyboard is behaving a bit weird the problem is in
> > the kernel as it will interpret 0x31 and 0x32 as the same key.
> 
> Thanks for the information! Please include that in follow-up
> commit-messages so others can see it as well. I don't think a report
> descriptor is needed, anymore.
> 
> This indeed explains the problem. We only track keys on the keycode
> level, not scancode level. Therefore, you get weird key-up or
> key-repeat events depending on the scan-order.
> 
> The nicest fix, obviously, is to blacklist keys that are not
> physically present on the keyboard. But I assume the keyboard reports

Hmm, somebody could still load keymap with duplicate keycodes though...

>
> either key depending on the model (standard vs. European). Given that
> this key is indeed special as we only have a single keycode for it, we
> probably need some quirk like yours. I'd like to hear Dmitry's
> comments on this, maybe he has had similar problems with non-HID
> keyboards.

Hmm, I do not think we have a good story for duplicate keycodes for
drivers that send entire state as opposed to just changed bits.

Thanks.

-- 
Dmitry