[PATCH v4] arm64: dts: rockchip: Fix SD card init on rk3399-nanopi4

[Christian Kohlschütter]

mmc/SD-card initialization may fail on NanoPi R4S with
"mmc1: problem reading SD Status register" /
"mmc1: error -110 whilst initialising SD card"
either on cold boot or after a reboot.

Moreover, the system would also sometimes hang upon reboot.

This is prevented by setting an explicit undervoltage protection limit
for the SD-card-specific vcc3v0_sd voltage regulator.

Set the undervoltage protection limit to 2.7V, which is the minimum
permissible SD card operating voltage.

Signed-off-by: Christian Kohlschütter <christian@xxxxxxxxxxxxxxxx>
---
arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi | 4 ++++
1 file changed, 4 insertions(+)
mode change 100644 => 100755 arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi

diff --git a/arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi b/arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi
old mode 100644
new mode 100755
index 8c0ff6c96e03..669c74ce4d13
--- a/arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi
+++ b/arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi
@@ -73,6 +73,10 @@ vcc3v0_sd: vcc3v0-sd {
		regulator-always-on;
		regulator-min-microvolt = <3000000>;
		regulator-max-microvolt = <3000000>;
+
+		// must be configured or SD card may fail to initialize occasionally
+		regulator-uv-protection-microvolt = <2700000>;
+
		regulator-name = "vcc3v0_sd";
		vin-supply = <&vcc3v3_sys>;
	};
-- 
2.36.1