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

[Tanmay Kathpalia]

Hi Peng,

Thank you for your feedback.

On 10/22/2025 8:46 AM, Peng Fan wrote:
> 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
> 


According to the SD specification for the Switch Function command (CMD6):

"If the card is initialized in 3.3V signal level, Default Speed and High 
Speed are assigned to function 0 and 1. Then support bits of function 2 
to 4 (SDR50, SDR104 and DDR50) are set to 0. If the card is initialized 
in 1.8V signal level, SDR and DDR modes are assigned from function 0 to 
function 4."

This allows the host to detect if the card is already operating at 1.8V 
signaling and proceed accordingly.

I have tested this approach with multiple SD cards from different 
vendors and capacities, including:

- Kingston Canvas Select Plus 16GB SDR104
- Samsung EVO Select 128GB SDR104
- Samsung Evo Plus 128GB SDR104

All of these cards worked reliably in my tests.

For further reference, similar discussions and solutions have been 
proposed in the Linux community:

https://lore.kernel.org/linux-mmc/a367a679-28f7-898a-c043-27df8c9c9aba@intel.com/
https://lore.kernel.org/linux-mmc/1506328144-13666-1-git-send-email-adrian.hunter@intel.com/

Thanks,
Tanmay