Re: [PATCH v2] HID: i2c-hid: Revert to await reset ACK before reading report descriptor

[Benjamin Tissoires]

On Mon, Apr 22, 2024 at 7:11 PM Linux regression tracking (Thorsten
Leemhuis) <regressions@xxxxxxxxxxxxx> wrote:
>
> On 31.03.24 20:24, Kenny Levinsen wrote:
> > In af93a167eda9, i2c_hid_parse was changed to continue with reading the
> > report descriptor before waiting for reset to be acknowledged.
> >
> > This has lead to two regressions:
>
> Lo! Jiri, Benjamin, quick question: is there a reason why this fix for a
> 6.8-rc1 regression after more than two and half weeks is not yet
> mainlined? Or is there some good reason why we should be should be extra
> cautious?

No special reasons I guess. Neither Jiri nor I have sent a HID update
for this rc cycle, so it's still there, waiting to be pushed.
I've been quite busy with BPF lately and dropped the ball slightly on
the HID maintainer side, but I'm sure we'll send the PR to Linus this
week or the next.

Cheers,
Benjamin


>
>
> Side note: I noticed this due to the tracking today, but I also saw a
> user that recently ran into the problem the quoted fix is supposed to
> resolve: https://social.lol/@major/112294923280815017
>
> Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
> --
> Everything you wanna know about Linux kernel regression tracking:
> https://linux-regtracking.leemhuis.info/about/#tldr
> If I did something stupid, please tell me, as explained on that page.
>
> #regzbot poke
>
> > 1. We fail to handle reset acknowledgement if it happens while reading
> >    the report descriptor. The transfer sets I2C_HID_READ_PENDING, which
> >    causes the IRQ handler to return without doing anything.
> >
> >    This affects both a Wacom touchscreen and a Sensel touchpad.
> >
> > 2. On a Sensel touchpad, reading the report descriptor this quickly
> >    after reset results in all zeroes or partial zeroes.
> >
> > The issues were observed on the Lenovo Thinkpad Z16 Gen 2.
> >
> > The change in question was made based on a Microsoft article[0] stating
> > that Windows 8 *may* read the report descriptor in parallel with
> > awaiting reset acknowledgement, intended as a slight reset performance
> > optimization. Perhaps they only do this if reset is not completing
> > quickly enough for their tastes?
> >
> > As the code is not currently ready to read registers in parallel with a
> > pending reset acknowledgement, and as reading quickly breaks the report
> > descriptor on the Sensel touchpad, revert to waiting for reset
> > acknowledgement before proceeding to read the report descriptor.
> >
> > [0]: https://learn.microsoft.com/en-us/windows-hardware/drivers/hid/plug-and-play-support-and-power-management
> >
> > Fixes: af93a167eda9 ("HID: i2c-hid: Move i2c_hid_finish_hwreset() to after reading the report-descriptor")
> > Signed-off-by: Kenny Levinsen <kl@xxxxxx>
> > ---
> >  drivers/hid/i2c-hid/i2c-hid-core.c | 13 ++++---------
> >  1 file changed, 4 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/hid/i2c-hid/i2c-hid-core.c b/drivers/hid/i2c-hid/i2c-hid-core.c
> > index 2df1ab3c31cc..72d2bccf5621 100644
> > --- a/drivers/hid/i2c-hid/i2c-hid-core.c
> > +++ b/drivers/hid/i2c-hid/i2c-hid-core.c
> > @@ -735,9 +735,12 @@ static int i2c_hid_parse(struct hid_device *hid)
> >       mutex_lock(&ihid->reset_lock);
> >       do {
> >               ret = i2c_hid_start_hwreset(ihid);
> > -             if (ret)
> > +             if (ret == 0)
> > +                     ret = i2c_hid_finish_hwreset(ihid);
> > +             else
> >                       msleep(1000);
> >       } while (tries-- > 0 && ret);
> > +     mutex_unlock(&ihid->reset_lock);
> >
> >       if (ret)
> >               goto abort_reset;
> > @@ -767,16 +770,8 @@ static int i2c_hid_parse(struct hid_device *hid)
> >               }
> >       }
> >
> > -     /*
> > -      * Windows directly reads the report-descriptor after sending reset
> > -      * and then waits for resets completion afterwards. Some touchpads
> > -      * actually wait for the report-descriptor to be read before signalling
> > -      * reset completion.
> > -      */
> > -     ret = i2c_hid_finish_hwreset(ihid);
> >  abort_reset:
> >       clear_bit(I2C_HID_RESET_PENDING, &ihid->flags);
> > -     mutex_unlock(&ihid->reset_lock);
> >       if (ret)
> >               goto out;
> >
>