linux-kernel - Re: [PATCH v2] HID: i2c-hid: goodix: Tie the reset line to true state of the regulator

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Message-ID: <nycvar.YFH.7.76.2107152150060.8253@cbobk.fhfr.pm>
Date:   Thu, 15 Jul 2021 21:50:21 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Douglas Anderson <dianders@...omium.org>
cc:     Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] HID: i2c-hid: goodix: Tie the reset line to true
 state of the regulator

On Fri, 25 Jun 2021, Douglas Anderson wrote:

> The regulator for the touchscreen could be:
> * A dedicated regulator just for the touchscreen.
> * A regulator shared with something else in the system.
> * An always-on regulator.
> 
> How we want the "reset" line to behave depends a bit on which of those
> three cases we're in. Currently the code is written with the
> assumption that it has a dedicated regulator, but that's not really
> guaranteed to be the case.
> 
> The problem we run into is that if we leave the touchscreen powered on
> (because someone else is requesting the regulator or it's an always-on
> regulator) and we assert reset then we apparently burn an extra 67 mW
> of power. That's not great.
> 
> Let's instead tie the control of the reset line to the true state of
> the regulator as reported by regulator notifiers. If we have an
> always-on regulator our notifier will never be called. If we have a
> shared regulator then our notifier will be called when the touchscreen
> is truly turned on or truly turned off.
> 
> Using notifiers like this nicely handles all the cases without
> resorting to hacks like pretending that there is no "reset" GPIO if we
> have an always-on regulator.
> 
> NOTE: if the regulator is on a shared line it's still possible that
> things could be a little off. Specifically, this case is not handled
> even after this patch:
> 1. Suspend goodix (send "sleep", goodix stops requesting regulator on)
> 2. Other regulator user turns off (regulator fully turns off).
> 3. Goodix driver gets notified and asserts reset.
> 4. Other regulator user turns on.
> 5. Goodix driver gets notified and deasserts reset.
> 6. Nobody resumes goodix.
> 
> With that set of steps we'll have reset deasserted but we will have
> lost the results of the I2C_HID_PWR_SLEEP from the suspend path. That
> means we might be in higher power than we could be even if the goodix
> driver thinks things are suspended. Presumably, however, we're still
> in better shape than if we were asserting "reset" the whole time. If
> somehow the above situation is actually affecting someone and we want
> to do better we can deal with it when we have a real use case.
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>

Applied, thanks Doug.

-- 
Jiri Kosina
SUSE Labs