obscure microsd detection issue between U-Boot and kernel

[Tim Harvey]

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)?

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?

Best Regards,

Tim