[PATCH v2 2/6] mmc: rtsx_usb_sdmmc: start card power-up at 3.3V

[Sean Rhodes <sean@starlabs.systems>]

Some tray-based readers keep SD_CD asserted even without a card. The
rtsx_usb_sdmmc driver now validates insertion with a minimal probe
sequence. That probe must start with the SD pads in 3.3V mode.

Like the old rts5139 driver (sd_init_power()), force the SD pads to
3.3V and tune the SD18 regulator to 3.3V before powering up the card.
This avoids spurious probe timeouts when the reader is left in 1.8V
from a previous UHS session.

Tested: Realtek RTS5129 (0bda:0129) + tray + Lexar 2TB SDXC
Tested: cold boot detects mmcblk0 (2026-02-24)
Tested: hotplug insert enumerates mmcblk0 (2026-02-23)
Signed-off-by: Sean Rhodes <sean@starlabs.systems>
---
 drivers/mmc/host/rtsx_usb_sdmmc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/mmc/host/rtsx_usb_sdmmc.c b/drivers/mmc/host/rtsx_usb_sdmmc.c
index ec3eeea78e95..6be98926387d 100644
--- a/drivers/mmc/host/rtsx_usb_sdmmc.c
+++ b/drivers/mmc/host/rtsx_usb_sdmmc.c
@@ -1108,6 +1108,11 @@ static int sd_power_on(struct rtsx_usb_sdmmc *host)
 	}
 	dev_dbg(sdmmc_dev(host), "%s\n", __func__);
 	rtsx_usb_init_cmd(ucr);
+	/* Start SD init at 3.3V, like the old rts5139 driver. */
+	rtsx_usb_add_cmd(ucr, WRITE_REG_CMD, SD_PAD_CTL,
+			 SD_IO_USING_1V8, SD_IO_USING_3V3);
+	rtsx_usb_add_cmd(ucr, WRITE_REG_CMD, LDO_POWER_CFG,
+			 TUNE_SD18_MASK, TUNE_SD18_3V3);
 	rtsx_usb_add_cmd(ucr, WRITE_REG_CMD, CARD_SELECT, 0x07, SD_MOD_SEL);
 	rtsx_usb_add_cmd(ucr, WRITE_REG_CMD, CARD_SHARE_MODE,
 			CARD_SHARE_MASK, CARD_SHARE_SD);
-- 
2.51.0