[PATCH RFC v2 3/5] arm64: dts: qcom: nord-embedded: Raise i2c18 pad drive strength

[Shawn Guo]

The i2c18 pins (gpio154/155) are configured for 2 mA. That is not
enough on nord-ride-embedded, where i2c18 is the audio control bus
carrying the TCA6408 expander, ADAU1979 ADC and PCM1681 DAC behind
TXS0102 level shifters. Transfers intermittently fail with arbitration
loss and unexpected start/stop conditions:

  geni_i2c 890000.i2c: Bus arbitration lost, clock line undriveable
  geni_i2c 890000.i2c: Bus proto err, noisy/unexpected start/stop
  adau1977 0-0031: probe with driver adau1977 failed with error -11
  reg-fixed-voltage regulator-5p0-dac: setup of GPIO (default) failed: -11

The failure became much more frequent with commit 8cb5c17be3a2
("i2c: qcom-geni: Restore the source-clock selection policy"), which
moves the i2c18 serial engine clock from 32 MHz to 19.2 MHz. That
commit only changes how often the marginal signalling shows up:
reverting it, or lowering the bus to 100 kHz, still leaves occasional
failures in a codec unbind/bind loop. With 4 mA the loop passes
reliably at either source clock rate.

nord-rrd uses the same level shifters and has not shown the failure
at 2 mA, but that appears to be margin rather than a guarantee. Since
this audio I2C arrangement is the reference design that customer
boards will likely follow, raise the default in the shared dtsi
instead of overriding it per board.

Signed-off-by: Shawn Guo <shengchao.guo@xxxxxxxxxxxxxxxx>
---
 arch/arm64/boot/dts/qcom/nord-embedded.dtsi | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/boot/dts/qcom/nord-embedded.dtsi b/arch/arm64/boot/dts/qcom/nord-embedded.dtsi
index 6dc4c871f41e..1ead642254e0 100644
--- a/arch/arm64/boot/dts/qcom/nord-embedded.dtsi
+++ b/arch/arm64/boot/dts/qcom/nord-embedded.dtsi
@@ -1221,7 +1221,7 @@ qup_i2c17_default: qup-i2c17-default-state {
 	qup_i2c18_default: qup-i2c18-default-state {
 		pins = "gpio154", "gpio155";
 		function = "qup2_se4";
-		drive-strength = <2>;
+		drive-strength = <4>;
 		bias-pull-up;
 	};
 
-- 
2.43.0