linux-kernel - Re: [PATCH] Input: elants_i2c: Properly handle the reset GPIO when power is off

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Message-ID: <Y3ayEc8sFCLahOT3@google.com>
Date:   Thu, 17 Nov 2022 14:13:37 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Benson Leung <bleung@...omium.org>,
        Johnny Chuang <johnny.chuang.emc@...il.com>,
        Scott Liu <scott.liu@....com.tw>, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Input: elants_i2c: Properly handle the reset GPIO when
 power is off

On Thu, Nov 17, 2022 at 12:38:23PM -0800, Douglas Anderson wrote:
> As can be seen in elants_i2c_power_off(), we want the reset GPIO
> asserted when power is off. The reset GPIO is active low so we need
> the reset line logic low when power is off to avoid leakage.
> 
> We have a problem, though, at probe time. At probe time we haven't
> powered the regulators on yet but we have:
>   devm_gpiod_get(&client->dev, "reset", GPIOD_OUT_LOW);
> 
> While that _looks_ right, it turns out that it's not. The
> GPIOD_OUT_LOW doesn't mean to init the GPIO to low. It means init the
> GPIO to "not asserted". Since this is an active low GPIO that inits it
> to be high.
> 
> Let's fix this to properly init the GPIO. Now after both probe and
> power off the state of the GPIO is consistent (it's "asserted" or
> level low).
> 
> Once we fix this, we can see that at power on time we no longer to
> assert the reset GPIO as the first thing. The reset GPIO is _always_
> asserted before powering on. Let's fix powering on to account for
> this.

I kind of like that elants_i2c_power_on() is self-contained and does the
full power sequence. Can we simply change devm_gpiod_get() to use
GPIOD_ASIS to avoid the momentary spike in reset line state (assuming
that the firmware initializes the reset line sanely because if it does
not we have much longer time where we are leaking into the controller)?

Thanks.

-- 
Dmitry