Date:   Tue, 7 Feb 2023 18:31:47 +0000
From:   Matthias Kaehlcke <mka@chromium.org>
To:     Douglas Anderson <dianders@chromium.org>
Cc:     Bjorn Andersson <andersson@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Jiri Kosina <jikos@kernel.org>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        linux-input@vger.kernel.org, linux-arm-msm@vger.kernel.org,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        devicetree@vger.kernel.org, Stephen Kitt <steve@sk2.org>,
        Konrad Dybcio <konrad.dybcio@linaro.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/7] HID: i2c-hid: goodix: Stop tying the reset line to
 the regulator
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References: <20230207024816.525938-1-dianders@chromium.org>
 <20230206184744.4.I085b32b6140c7d1ac4e7e97b712bff9dd5962b62@changeid>
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On Mon, Feb 06, 2023 at 06:48:13PM -0800, Douglas Anderson wrote:
> In commit 18eeef46d359 ("HID: i2c-hid: goodix: Tie the reset line to
> true state of the regulator"), we started tying the reset line of
> Goodix touchscreens to the regulator.
> 
> The primary motivation for that patch was some pre-production hardware
> (specifically sc7180-trogdor-homestar) where it was proposed to hook
> the touchscreen's main 3.3V power rail to an always-on supply. In such
> a case, when we turned "off" the touchscreen in Linux it was bad to
> assert the "reset" GPIO because that was causing a power drain. The
> patch accomplished that goal and did it in a general sort of way that
> didn't require special properties to be added in the device tree for
> homestar.
> 
> It turns out that the design of using an always-on power rail for the
> touchscreen was rejected soon after the patch was written and long
> before sc7180-trogdor-homestar went into production. The final design
> of homestar actually fully separates the rail for the touchscreen and
> the display panel and both can be powered off and on. That means that
> the original motivation for the feature is gone.
> 
> There are 3 other users of the goodix i2c-hid driver in mainline.
> 
> I'll first talk about 2 of the other users in mainline: coachz and
> mrbland. On both coachz and mrbland the touchscreen power and panel
> power _are_ shared. That means that the patch to tie the reset line to
> the true state of the regulator _is_ doing something on those
> boards. Specifically, the patch reduced power consumption by tens of
> mA in the case where we turned the touchscreen off but left the panel
> on. Other than saving a small bit of power, the patch wasn't truly
> necessary. That being said, even though a small bit of power was saved
> in the state of "panel on + touchscreen off", that's not actually a
> state we ever expect to be in, except perhaps for very short periods
> of time at boot or during suspend/resume. Thus, the patch is truly not
> necessary. It should be further noted that, as documented in the
> original patch, the current code still didn't optimize power for every
> corner case of the "shared rail" situation.
> 
> The last user in mainline was very recently added: evoker. Evoker is
> actually the motivation for me removing this bit of code. It turns out
> that for evoker we need to manage a second power rail for IO to the
> touchscreen. Trying to fit the management of this IO rail into the
> regulator notifiers turns out to be extremely hard. To avoid lockdep
> splats you shouldn't enable/disable other regulators in regulator
> notifiers and trying to find a way around this was going to be fairly
> difficult.
> 
> Given the lack of any true motivation to tie the reset line to the
> regulator, lets go back to the simpler days and remove the code. This
> is, effectively, a revert of commit bdbc65eb77ee ("HID: i2c-hid:
> goodix: Fix a lockdep splat"), commit 25ddd7cfc582 ("HID: i2c-hid:
> goodix: Use the devm variant of regulator_register_notifier()"), and
> commit 18eeef46d359 ("HID: i2c-hid: goodix: Tie the reset line to true
> state of the regulator").
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.org>

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