linux-kernel - Re: [PATCH v3 1/4] arm64: dts: qcom: sc7180: Bump up trogdor ts_reset

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Date:   Sat, 10 Dec 2022 12:05:38 +0100
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>,
        Bjorn Andersson <andersson@...nel.org>
Cc:     swboyd@...omium.org, Dmitry Torokhov <dmitry.torokhov@...il.com>,
        mka@...omium.org, Yunlong Jia <ecs.beijing2022@...il.com>,
        linux-arm-msm@...r.kernel.org, linux-input@...r.kernel.org,
        Andy Gross <agross@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] arm64: dts: qcom: sc7180: Bump up trogdor
 ts_reset_l drive strength



On 9.12.2022 18:12, Douglas Anderson wrote:
> On at least one board (pazquel360) the reset line for the touchscreen
> was scoped and found to take almost 2 ms to fall when we drove it
> low. This wasn't great because the Linux driver for the touchscreen
> (the elants_i2c driver) thinks it can do a 500 us reset pulse. If we
> bump the drive strength to 8 mA then the reset line went down in ~421
> us.
> 
> NOTE: we could apply this fix just for pazquel360, but:
> * Probably other trogdor devices have similar timings and it's just
>   that nobody has noticed it before.
> * There are other trogdor boards using the same elan driver that tries
>   to do 500 us reset pulses.
> * Bumping the drive strength to 8mA across the board won't hurt. This
>   isn't a high speed signal or anything.
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Reviewed-by: Matthias Kaehlcke <mka@...omium.org>
> ---
Reviewed-by: Konrad Dybcio <konrad.dybcio@...aro.org>

Konrad
> 
> (no changes since v1)
> 
>  arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi b/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi
> index f1defb94d670..ff1c7aa6a722 100644
> --- a/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi
> +++ b/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi
> @@ -1376,7 +1376,15 @@ ts_reset_l: ts-reset-l-state {
>  		pins = "gpio8";
>  		function = "gpio";
>  		bias-disable;
> -		drive-strength = <2>;
> +
> +		/*
> +		 * The reset GPIO to the touchscreen takes almost 2ms to drop
> +		 * at the default drive strength. When we bump it up to 8mA it
> +		 * falls in under 500us. We want this to be fast since the Elan
> +		 * datasheet (and any drivers written based on it) talk about using
> +		 * a 500 us reset pulse.
> +		 */
> +		drive-strength = <8>;
>  	};
>  
>  	sdc1_on: sdc1-on-state {