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

[Judith Mendez]

Hi Tanmay, all,

On 10/21/25 3:45 PM, 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.

This implementation broke am65 IDK board SD card boot, I have still
to check why this breaks only one board, potentially there might
be a quirk only for this board... But for now sending the question
in case anyone might have an idea what is going on & save me some
time.

~ Judith