From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>

Add a ramp delay of 60 uV/us to the vqmmc_sdhi0 voltage regulator to
fix UHS-I SD card detection failures.

Measurements on CN78 pin 4 showed the actual voltage ramp time to be
21.86ms when switching between 3.3V and 1.8V. A 25ms ramp delay has
been configured to provide adequate margin. The calculation is based
on the voltage delta of 1.5V (3.3V - 1.8V):
  1500000 uV / 60 uV/us = 25000 us (25ms)

Prior to this patch, UHS-I cards failed to initialize with:

  [   32.723914] mmc0: error -110 whilst initialising SD card

After this patch, UHS-I cards are properly detected on SD0:

  [   28.379122] mmc0: new UHS-I speed SDR104 SDXC card at address aaaa
  [   28.387239] mmcblk0: mmc0:aaaa SR64G 59.5 GiB

Fixes: d065453e5ee09("arm64: dts: renesas: rzt2h-rzn2h-evk: Enable SD card slot")
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
---
 arch/arm64/boot/dts/renesas/rzt2h-n2h-evk-common.dtsi | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/boot/dts/renesas/rzt2h-n2h-evk-common.dtsi b/arch/arm64/boot/dts/renesas/rzt2h-n2h-evk-common.dtsi
index 510399febf29..f87c2492f414 100644
--- a/arch/arm64/boot/dts/renesas/rzt2h-n2h-evk-common.dtsi
+++ b/arch/arm64/boot/dts/renesas/rzt2h-n2h-evk-common.dtsi
@@ -53,6 +53,7 @@ vqmmc_sdhi0: regulator-vqmmc-sdhi0 {
 		regulator-max-microvolt = <3300000>;
 		gpios-states = <0>;
 		states = <3300000 0>, <1800000 1>;
+		regulator-ramp-delay = <60>;
 	};
 #endif
 
-- 
2.52.0