obscure microsd detection issue between U-Boot and kernel

[Michael Walle]

On Tue Jun 4, 2024 at 9:47 AM CEST, Christian Loehle wrote:
> On 6/3/24 22:28, Tim Harvey wrote:
> > 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.
>
> Yes, the only safe way to use UHS-I really anyway.

I can second that. And also keep in mind that the actual supply
voltage must be below that threshold. Thus, the power-off time also
depends on the capacitance on that supply line after the power load
switch. There are switches with dedicated output discharge circuits
built-in.

That being said, from my experience there are cards which will work
when switching back from 1V8 to 3V3 signalling and some don't. I
haven't digged deeper into that topic, though.

-michael

> 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

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