linux-kernel - Re: [PATCH 5/5] Input: elants_i2c: Delay longer with reset asserted

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Message-ID: <CAD=FV=Vtar2gGyeKdzttGZyUCMJOFd70EWqkbx3iL2rMTHN1ig@mail.gmail.com>
Date:   Thu, 8 Dec 2022 17:49:25 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Bjorn Andersson <andersson@...nel.org>, mka@...omium.org,
        swboyd@...omium.org, linux-arm-msm@...r.kernel.org,
        linux-input@...r.kernel.org,
        Yunlong Jia <ecs.beijing2022@...il.com>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Johnny Chuang <johnny.chuang.emc@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] Input: elants_i2c: Delay longer with reset asserted

Hi,

On Thu, Dec 8, 2022 at 5:48 PM Dmitry Torokhov
<dmitry.torokhov@...il.com> wrote:
>
> On Thu, Dec 08, 2022 at 05:38:28PM -0800, Dmitry Torokhov wrote:
> > On Thu, Dec 08, 2022 at 11:20:06AM -0800, Douglas Anderson wrote:
> > > The elan touchscreen datasheet says that the reset GPIO only needs to
> > > be asserted for 500us in order to reset the regulator. The problem is
> > > that some boards need a level shifter between the signals on the GPIO
> > > controller and the signals on the touchscreen. All of these extra
> > > components on the line can slow the transition of the signals. On one
> > > board, we measured the reset line and saw that it took almost 1.8ms to
> > > go low. Even after we bumped up the "drive strength" of the signal
> > > from the default 2mA to 8mA we still saw it take 421us for the signal
> > > to go low.
> > >
> > > In order to account for this we let's lengthen the amount of time that
> > > we keep the reset asserted. Let's bump it up from 500us to 5000us.
> > > That's still a relatively short amount of time and is much safer.
> > >
> > > It should be noted that this fixes real problems. Case in point:
> > > 1. The touchscreen power rail may be shared with another device (like
> > >    an eDP panel). That means that at probe time power might already be
> > >    on.
> > > 2. In probe we grab the reset GPIO and assert it (make it low).
> > > 3. We turn on power (a noop since it was already on).
> > > 4. We wait 500us.
> > > 5. We deassert the reset GPIO.
> > >
> > > With the above case and only a 500us delay we saw only a partial reset
> > > asserted, which is bad. Giving it 5ms is overkill but feels safer in
> > > case someone else has a different level shifter setup.
> > >
> > > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> >
> > Applied, thank you.
>
> Unapplied ;) I guess we should follow kernel test robot's advise and
> switch to using usleep_range().

Crud. I'll send a v2 right away.

-Doug