Re: [PATCH 1/2] dt-bindings: input: goodix: Add powered-in-suspend property

From: Doug Anderson <dianders@chromium.org>
To: Fei Shao <fshao@chromium.org>
Cc: Jeff LaBundy <jeff@labundy.com>,
	Benjamin Tissoires <benjamin.tissoires@redhat.com>,
	Rob Herring <robh+dt@kernel.org>,
	linux-mediatek <linux-mediatek@lists.infradead.org>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	devicetree@vger.kernel.org, linux-input@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: input: goodix: Add powered-in-suspend property
Date: Wed, 19 Apr 2023 07:38:13 -0700	[thread overview]
Message-ID: <CAD=FV=U_i26a8uJYmqYf6PUgmTUgmEB5L2DkVga0zDX_iDcGQg@mail.gmail.com> (raw)
In-Reply-To: <CAC=S1ngBt9DmBobMkQXWhqE1UUxFv2U6iFd42nT=1N7r8+pFUg@mail.gmail.com>

Hi,

On Wed, Apr 19, 2023 at 3:44 AM Fei Shao <fshao@chromium.org> wrote:
>
> Hi Jeff,
>
> On Wed, Apr 19, 2023 at 8:21 AM Jeff LaBundy <jeff@labundy.com> wrote:
> >
> > Hi Fei,
> >
> > On Tue, Apr 18, 2023 at 08:49:51PM +0800, Fei Shao wrote:
> > > We observed that on Chromebook device Steelix, if Goodix GT7375P
> > > touchscreen is powered in suspend (because, for example, it connects to
> > > an always-on regulator) and with the reset GPIO asserted, it will
> > > introduce about 14mW power leakage.
> > >
> > > This property is used to indicate that the touchscreen is powered in
> > > suspend. If it's set, the driver will stop asserting the reset GPIO in
> > > power-down, and it will do it in power-up instead to ensure that the
> > > state is always reset after resuming.
> > >
> > > Signed-off-by: Fei Shao <fshao@chromium.org>
> > > ---
> >
> > This is an interesting problem; were you able to root-cause why the silicon
> > exhibits this behavior? Simply asserting reset should not cause it to draw
> > additional power, let alone 14 mW. This almost sounds like a back-powering
> > problem during suspend.
> >
> There was a fix for this behavior before so I didn't dig into it on
> the silicon side.
> I can ask internally and see if we can have Goodix to confirm this is
> a known HW erratum.

Certainly it doesn't hurt to check, but it's not really that shocking
to me that asserting reset could cause a power draw on some hardware.
Reset puts hardware into a default state and that's not necessarily
low power. I guess ideally hardware would act like it's "off" when
reset is asserted and then then init to the default state on the edge
as reset was deasserted, but I not all hardware is designed in an
ideal way.

> > If this is truly expected behavior, is it sufficient to use the always_on
> > constraint of the relevant regulator(s) to make this decision as opposed to
> > introducing a new property?
> >
> That sounds good to me. IIUC, for the existing designs, the boards
> that would set this property would also exclusively set
> `regulator-always-on` in their supply, so that should suffice.
> Let me revise the patch. Thanks!

Yeah, I thought about this too and talked about it in my original
reply. It doesn't handle the shared-rail case, but then again neither
does ${SUBJECT} patch. ...then I guess the only argument against it is
my argument that the regulator could be marked "always-on" in the
device tree but still turned off by an external entity (PMIC or EC) in
S3. In theory this should be specified by
"regulator-state-(standby|mem|disk)", but I could believe it being
tricky to figure out (what if a parent regulator gets turned off
automatically but the child isn't explicit?). Specifically, if a
regulator is always-on but somehow gets shut off in suspend then we
_do_ want to assert reset (active low) during suspend, otherwise we'll
have a power leak through the reset GPIO... :-P

...so I guess I'll continue to assert that I don't think peeking at
the regulator's "always-on" property is the best way to go. If
everyone else disagrees with me then I won't stand in the way, but IMO
the extra property like Fei's patch adds is better.

[1] https://lore.kernel.org/r/CAD=FV=V8ZN3969RrPu2-zZYoEE=LDxpi8K_E8EziiDpGOSsq1w@mail.gmail.com