linux-kernel - Re: [PATCH v2 1/2] dt-bindings: input: touchscreen: goodix: Add no-reset-pull-up property

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Message-ID: <w65v3filerdiuimbnrcmislknzeug2f6nont2r4atzq2p2w2nv@7iskwi7g2u2e>
Date: Tue, 29 Apr 2025 14:32:43 -0700
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Esben Haabendal <esben@...nix.com>
Cc: Conor Dooley <conor@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Hans de Goede <hdegoede@...hat.com>, linux-input@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] dt-bindings: input: touchscreen: goodix: Add
 no-reset-pull-up property

On Tue, Apr 29, 2025 at 09:02:14PM +0200, Esben Haabendal wrote:
> "Dmitry Torokhov" <dmitry.torokhov@...il.com> writes:
> 
> > On Tue, Apr 29, 2025 at 05:37:34PM +0200, Esben Haabendal wrote:
> >> "Conor Dooley" <conor@...nel.org> writes:
> >>
> >> > On Tue, Apr 29, 2025 at 11:56:11AM +0200, Esben Haabendal wrote:
> >> >> This should be added for boards where there is no pull-up on the reset pin,
> >> >> as the driver will otherwise switch the reset signal to high-impedance to
> >> >> save power, which obviously not safe without pull-up.
> >> >>
> >> >> Signed-off-by: Esben Haabendal <esben@...nix.com>
> >> >> ---
> >> >>  Documentation/devicetree/bindings/input/touchscreen/goodix.yaml | 4 ++++
> >> >>  1 file changed, 4 insertions(+)
> >> >>
> >> >> diff --git a/Documentation/devicetree/bindings/input/touchscreen/goodix.yaml b/Documentation/devicetree/bindings/input/touchscreen/goodix.yaml
> >> >> index eb4992f708b70fef93bd4b59b9565123f7c6ad5d..21ac13046b6e021eeb403d854aabc945801dd29f 100644
> >> >> --- a/Documentation/devicetree/bindings/input/touchscreen/goodix.yaml
> >> >> +++ b/Documentation/devicetree/bindings/input/touchscreen/goodix.yaml
> >> >> @@ -45,6 +45,10 @@ properties:
> >> >>    reset-gpios:
> >> >>      maxItems: 1
> >> >>
> >> >> +  goodix,no-reset-pull-up:
> >> >> +    type: boolean
> >> >> +    description: There is no pull-up on reset pin
> >> >
> >> > I have to wonder, why are these system using the reset property if the
> >> > reset is not usable? Shouldn't the property be omitted?
> >>
> >> The reset are fully functional. It just have to be controlled in
> >> push-pull mode.
> >>
> >> Because of the lack of external pull-up, configuring the reset gpio as
> >> input (to save power) puts the reset pin in an unknown state.
> >
> > How much power do we save by doing this? I don't recall other drivers
> > trying to switch reset GPIO into input mode after performing reset...
> 
> I don't know.  I also don't recall seeing this in other drivers.  But as
> not knowing how much power it is, I did not feel comofortable proposing
> to remove it.

Hmm, setting RESET line to "input" was supposed to be removed in 2015:

https://lore.kernel.org/all/1473530257-7495-5-git-send-email-irina.tirdea@intel.com/

And I even said that I applied that patch, but I can't find it. And
it's been 10 years so I have no idea... The wording about power savings
seem to have come from Hans, maybe he knows/remembers more?

But I really wonder if we are not better off keeping reset line in
"inactive" output state, like every other driver does.

Thanks.

-- 
Dmitry