[PATCH] mmc: sd: Handle UHS-I voltage signaling without power cycle

[Peng Fan]

Hi Tanmay,

On Tue, Oct 21, 2025 at 01:45:26PM -0700, Tanmay Kathpalia wrote:
>Some boards have SD card connectors where the power rail cannot be switched
>off by the driver. However there are various circumstances when a card
>might be re-initialized, such as after system resume, warm re-boot, or
>error handling. However, a UHS card will continue to use 1.8V signaling
>unless it is power cycled.
>
>If the card has not been power cycled, it may still be using 1.8V
>signaling. According to the SD spec., the Bus Speed Mode (function group 1)
>bits 2 to 4 are zero if the card is initialized at 3.3V signal level. Thus
>they can be used to determine if the card has already switched to 1.8V
>signaling. Detect that situation and try to initialize a UHS-I (1.8V)
>transfer mode.

Actually a power cycle or reset is required to make sure SD card to work
correctly. Some SD cards may work in your case, some SD cards might not work,
without a power cycle or reset.

Thanks,
Peng

>
>Signed-off-by: Tanmay Kathpalia <tanmay.kathpalia at altera.com>