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.
