obscure microsd detection issue between U-Boot and kernel

[Tim Harvey]

On Mon, Jun 3, 2024 at 1:18 AM Christian Loehle
<christian.loehle at 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.

Best Regards,

Tim