[PATCH v3] 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.

While using a limit of 3V seems to work, an additional safety buffer
should prevent accidental tripping, preventing a system hang.

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 initialized or SD card may fail to initialize / system may hang
+		regulator-uv-protection-microvolt = <2700000>;
+
 		regulator-name = "vcc3v0_sd";
 		vin-supply = <&vcc3v3_sys>;
 	};
-- 
2.36.1