OK, this took me a while to figure out.
When no undervoltage limit is configured, I can reliably trigger the initialization bug upon boot.
When the limit is set to 3.0V, it rarely occurs, but just after I send the v3 patch, I was able to reproduce...
> Am 15.07.2022 um 19:12 schrieb Christian Kohlschütter <christian@xxxxxxxxxxxxxxxx>:
>
> 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
