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From:   Douglas Anderson <dianders@chromium.org>
To:     Bjorn Andersson <andersson@kernel.org>
Cc:     swboyd@chromium.org, Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        mka@chromium.org, Yunlong Jia <ecs.beijing2022@gmail.com>,
        linux-arm-msm@vger.kernel.org,
        Konrad Dybcio <konrad.dybcio@linaro.org>,
        linux-input@vger.kernel.org,
        Douglas Anderson <dianders@chromium.org>,
        Andy Gross <agross@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Rob Herring <robh+dt@kernel.org>, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH v3 1/4] arm64: dts: qcom: sc7180: Bump up trogdor ts_reset_l drive strength
Date:   Fri,  9 Dec 2022 09:12:37 -0800
Message-Id: <20221209091234.v3.1.I39c387f1e3176fcf340039ec12d54047de9f8526@changeid>
In-Reply-To: <20221209171240.1614904-1-dianders@chromium.org>
References: <20221209171240.1614904-1-dianders@chromium.org>
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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@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
---

(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 {
-- 
2.39.0.rc1.256.g54fd8350bd-goog