linux-kernel - Re: [PATCH 4/6] HID: i2c-hid: elan: fix reset suspend current leakage

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Date: Tue, 23 Apr 2024 13:37:14 -0700
From: Doug Anderson <dianders@...omium.org>
To: Johan Hovold <johan+linaro@...nel.org>
Cc: Jiri Kosina <jikos@...nel.org>, Benjamin Tissoires <benjamin.tissoires@...hat.com>, 
	Dmitry Torokhov <dmitry.torokhov@...il.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio <konrad.dybcio@...aro.org>, 
	Linus Walleij <linus.walleij@...aro.org>, linux-input@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-arm-msm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 4/6] HID: i2c-hid: elan: fix reset suspend current leakage

Hi,

On Tue, Apr 23, 2024 at 6:46 AM Johan Hovold <johan+linaro@...nel.org> wrote:
>
> @@ -87,12 +104,14 @@ static int i2c_hid_of_elan_probe(struct i2c_client *client)
>         ihid_elan->ops.power_up = elan_i2c_hid_power_up;
>         ihid_elan->ops.power_down = elan_i2c_hid_power_down;
>
> -       /* Start out with reset asserted */
> -       ihid_elan->reset_gpio =
> -               devm_gpiod_get_optional(&client->dev, "reset", GPIOD_OUT_HIGH);
> +       ihid_elan->reset_gpio = devm_gpiod_get_optional(&client->dev, "reset",
> +                                                       GPIOD_ASIS);

I'm not a huge fan of this part of the change. It feels like the GPIO
state should be initialized by the probe function. Right before we
call i2c_hid_core_probe() we should be in the state of "powered off"
and the reset line should be in a consistent state. If
"no_reset_on_power_off" then it should be de-asserted. Else it should
be asserted.

I think GPIOD_ASIS doesn't actually do anything useful for you, right?
i2c_hid_core_probe() will power on and the first thing that'll happen
there is that the reset line will be unconditionally asserted.

Having this as "GPIOD_ASIS" makes it feel like the kernel is somehow
able to maintain continuity of this GPIO line from the BIOS state to
the kernel, but I don't think it can. I've looked at the "GPIOD_ASIS"
property before because I've always wanted the ability to have GPIOs
that could more seamlessly transition their firmware state to their
kernel state. I don't think the API actually allows it. The fact that
GPIO regulators don't support this seamless transition (even though it
would be an obvious feature to add) supports my theory that the API
doesn't currently allow it. It may be possible to make something work
on some implementations but I think it's not guaranteed.

Specifically, the docs say:

* GPIOD_ASIS or 0 to not initialize the GPIO at all. The direction must be set
  later with one of the dedicated functions.

So that means that you can't read the pin without making it an input
(which might change the state if it was previously driving a value)
and you can't write the pin without making it an output and choosing a
value to set it to. Basically grabbing a pin with "asis" doesn't allow
you to do anything with it--it just claims it and doesn't let anyone
else have it.

-Doug