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

[Kathpalia, Tanmay]

On 5/14/2026 5:09 AM, Judith Mendez wrote:
> 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.
>
>

Thanks for reporting this. I had a deeper look at the code and found
an issue that is likely the root cause of the failure on am65 IDK.

During a normal cold boot with a UHS-capable host and UHS card, the
voltage switch to 1.8V is already performed inside sd_send_op_cond()
as part of the ACMD41 handshake, and this is recorded in mmc->ocr via
the S18R bit. By the time mmc_startup() calls sd_get_capabilities(),
the card is already operating at 1.8V and reports UHS bus speed modes,
causing mmc_sd_card_using_v18() to return true. The warm-reboot
recovery path then fires unconditionally and switches the host voltage
a second time, which on boards like am65 IDK can leave the controller
in a bad state.

The fix is to guard the recovery path so it only activates when the
voltage switch was not already performed during the current session:

-        if (mmc_sd_card_using_v18(mmc)) {
+        if (!(mmc->ocr & OCR_S18R) && mmc_sd_card_using_v18(mmc)) {

Could you please try this change and let me know if it resolves the
failure on am65 IDK?

I will send the patch for review on the mailing list.

Regards,
Tanmay