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

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Message-ID: <Y5J1wY/TzbU6BheD@google.com>
Date:   Thu, 8 Dec 2022 23:39:45 +0000
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Bjorn Andersson <andersson@...nel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        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

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

nit: s/we let's/we/ || s/we let's/let's/

no need to re-spin just for this

> 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>

Reviewed-by: Matthias Kaehlcke <mka@...omium.org>