linux-kernel - Re: [PATCH 2/2] arm64: dts: qcom: sc8280xp-x13s: Fix/enable touchscreen

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Message-ID: <20240126163646.GR2936378@hu-bjorande-lv.qualcomm.com>
Date: Fri, 26 Jan 2024 08:36:46 -0800
From: Bjorn Andersson <quic_bjorande@...cinc.com>
To: Johan Hovold <johan@...nel.org>
CC: Daniel Thompson <daniel.thompson@...aro.org>,
        Dmitry Torokhov
	<dmitry.torokhov@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof
 Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley
	<conor+dt@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Jiri Kosina <jikos@...nel.org>, Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Johan Hovold
	<johan+linaro@...nel.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-input@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Subject: Re: [PATCH 2/2] arm64: dts: qcom: sc8280xp-x13s: Fix/enable
 touchscreen

On Fri, Jan 26, 2024 at 05:23:30PM +0100, Johan Hovold wrote:
> On Fri, Jan 26, 2024 at 06:53:46AM -0800, Bjorn Andersson wrote:
> > On Fri, Jan 26, 2024 at 03:31:02PM +0100, Johan Hovold wrote:
> > > On Fri, Jan 26, 2024 at 01:02:32PM +0000, Daniel Thompson wrote:
> 
> > > > In short it looks like the delays make the difference and, even a short
> > > > delay, can fix the problem.
> > > 
> > > Right, but since the suppliers are left enabled by the bootloader (and
> > > never disabled by the kernel), that only begs the question of why this
> > > makes a difference.
> > 
> > You're right, the supply is kept on by other things, so this isn't the
> > problem.
> > 
> > > Without the delay, the other HID devices are probing (successfully)
> > > slightly before, but essentially in parallel with the touchscreen while
> > > using the same resources. Is that causing trouble somehow?
> > 
> > The difference to those other HID devices is GPIO 99 - the reset pin,
> > which is configured pull down input from boot - i.e. the chip is held in
> > reset.
> > 
> > When the HID device is being probed, pinctrl applies &ts0_default starts
> > driving it high, bringing the device out of reset. But insufficient time
> > is given for the chip to come up so the I2C read fails.
> 
> Ah, that's it.
> 
> You should drop that 'output-high' from the pin config as part of this
> patch to avoid toggling the reset line twice at boot.
> 

Sounds reasonable, let's fix that while we're at it...

> Looks like we have the same problem on the CRD as well. There the
> touchscreen still works, possibly because it has been enabled by the
> boot firmware or simply because that touchscreen can handle a shorter
> delay.
> 

I only poke the CRD remotely, forgot that it had touchscreen. Let's
bake a patch for that as well...

> Where exactly did you find those delay values in the ACPI tables? I
> couldn't seem to find anything in the decompiled DSDT.
> 

The PEP sequence for the touchscreen device.

Regards,
Bjorn

> > If you later try to probe again, 200ms has elapsed since the reset was
> > deasserted (driven high).
> 
> Right.
> 
> Johan