Re: obscure microsd detection issue between U-Boot and kernel

[Christian Loehle <christian.loehle@arm.com>]

On 6/3/24 22:28, Tim Harvey wrote:
> On Mon, Jun 3, 2024 at 1:18 AM Christian Loehle
> <christian.loehle@arm.com> wrote:
>>
>> On 5/31/24 21:47, Tim Harvey wrote:
>>> Greetings,
>>>
>>> I'm seeing an issue on an imx8mm board (imx8mm-venice-gw73xx) where
>>> for a specific set of microsd cards if I have accessed the microsd in
>>> U-Boot with UHS/1.8V the kernel will not recognize that microsd when
>>> scanning.
>>>
>>> The issue does not occur with all microsd cards but seems to appear
>>> with a large sample size of a specific card/model (Kingston SDC32 32GB
>>> SDR104 card). I do not see a signal integrity issue on the scope.
>>>
>>> Instrumenting the kernel the issue is that the host reports a CRC
>>> error as soon as the first mmc_send_if_cond call which occurs in
>>> mmc_rescan_try_freq.
>>>
>>> I can avoid the issue by either not accessing the microsd in U-Boot or
>>> by disabling UHS/1.8V mode in U-Boot therefore what I think is
>>> happening is that U-Boot leaves the card in UHS/1.8V signalling mode
>>> and when the kernel scans it sets the voltage back to 3.3V
>>> standard/default and default timings then issues its clock cycles to
>>> 'reset' the card and the card does not recognize the reset. I'm
>>> wondering if this is because the reset is done via clock cycles after
>>> the kernel has set the I/O voltage back to 3.3V when perhaps the card
>>> is still in 1.8V mode (although I don't see how that would cause an
>>> issue)?
>>
>> It will cause an issue for many cards and might break some cards.
>>
>>>
>>> Is there some sort of MMC 'reset' I can/should do in U-Boot before
>>> booting the kernel? Has anyone encountered anything like this before?
>>
>> There is no 'switching back' to 3.3V signalling from UHS 1.8V.
>> The only way this can be done is therefore a full power-off.
>> Is that done correctly for your system?
>> AFAIR spec dictates 500ms of <0.5V on VCC. Note that  driving CLK/signal
>> lines can also sustain the card somewhat, as leakage is only limited
>> within operating voltage.
> 
> Hi Christian,
> 
> Are you saying the only way to properly reset from 1.8V is to have a
> VDD supply on the microSD card that can be turned off before booting
> to Linux? We have never had that before and never encountered
> something like this.

Yes, the only safe way to use UHS-I really anyway.
You could disable UHS for u-boot but that still leaves (potentially)
problematic warm-reboots of the board.
Having a (software-controlled) switchable regulator on SD VCC is pretty
common for this reason and you should be able to find it in most dts
for host controllers with sd-uhs-* property.
I'm afraid that the relevant spec section isn't available in the
simplified version.

Kind Regards,
Christian