arm64: dts: qcom: sc7180: Bump up trogdor ts_reset_l drive strength

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>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221209091234.v3.1.I39c387f1e3176fcf340039ec12d54047de9f8526@changeid

diff --git a/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi b/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi
index d134d17..670a7d1 100644 (file)
--- a/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi
+++ b/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi
@@ -1394,7 +1394,15 @@ ap_spi_fp: &spi10 {
                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 {