[PATCH] imx: kontron-sl-mx8mm: Fix SD card IO voltage level

[Frieder Schrempf]

On 25.01.23 19:02, Marek Vasut wrote:
> On 1/25/23 18:30, Frieder Schrempf wrote:
>> Hi Marek,
> 
> Hi,
> 
>> On 25.01.23 18:15, Marek Vasut wrote:
>>> On 1/25/23 14:41, Frieder Schrempf wrote:
>>>> From: Frieder Schrempf <frieder.schrempf at kontron.de>
>>>
>>> Subject tags should be ARM: dts: imx:
>>
>> Ok, how do I know? Because "git log arch/arm/dts" shows me that
>> "<platform>: <board>:" is used quite often and it's what I used in the
>> past. But I'm happy to change it if "ARM: dts: imx:" is the way the
>> prefix should look like.
> 
> I just do what Linux does to keep things aligned.

Linux uses "arm64: dts:", but this doesn't match the directory structure
in U-Boot. Anyway keeping it the same as in Linux is probably best as
lots of DT patches are ported back and forth between the Linux and
U-Boot tree.

> 
>>>> The LDO5 of the PCA9450 PMIC can be switched between two different
>>>> voltage settings (defaulting to 1.8V and 3.3V) using an external
>>>> signal SD_VSEL that is connected to the VSELECT signal of the SD
>>>> card interface.
>>>>
>>>> As the regulator driver can't deal with both LDO registers (LDO5CTRL_H
>>>> and LDO5CTRL_L) it only uses one of them, which means reading the
>>>> voltage from the regulator can potentially return a value that does not
>>>> reflect the actual state of the LDO5 output.
>>>
>>> Can you fix the regulator driver ?
>>
>> So far, I didn't see a way how to do it, but with your suggestion below
>> it might work.
>>
>>>
>>>> In our case, after booting U-Boot we read 1.8V from the regulator
>>>> while in fact as the VSELECT signal is still low the regulator outputs
>>>> 3.3V. This confusion causes the MMC driver to think it is dealing with
>>>> a 1.8V-only device. This in turn leads to SD cards being addressed
>>>> with 1.8V IO levels even if UHS support is not available or disabled.
>>>>
>>>> Some cards with UHS support still work even if they are addressed
>>>> with 1.8V levels in non-UHS modes, but a lot of cards also fail with
>>>> timeout errors like:
>>>>
>>>>     Card did not respond to voltage select! : -110
>>>>
>>>> As a workaorund we disable the vqmmc regulator for now so we can make
>>>> sure no wrong values are read from the regulator. The switching
>>>> between 1.8V and 3.3V still works as the ESDHC driver sets the
>>>> VSELECT signal accordingly.
>>>>
>>>> Signed-off-by: Frieder Schrempf <frieder.schrempf at kontron.de>
>>>> ---
>>>> By the way: I suspect that other boards using the PCA9450 might also
>>>> be affected
>>>> by this. I didn't find a nice generic solution so far. It would be
>>>> possible to
>>>> patch the pca9450 driver to use the PCA9450_LDO5CTRL_L instead of the
>>>> PCA9450_LDO5CTRL_H register for LDO5. This would fix this particular
>>>> case,
>>>> but still not the root problem of the regulator driver returning wrong
>>>> values.
>>>> So if anyone got some idea how to properly handle this, let me know.
>>>> The same issue is also present in Linux. While I didn't notice any
>>>> problems
>>>> with the SD card being addressed with incorrect voltage levels so far,
>>>> reading the regulator doesn't return the correct value if VSELECT is
>>>> low.
>>>
>>> Configure VSELECT pinmux such that it is a function, as it is right now,
>>> MX8MP_IOMUXC_GPIO1_IO04__USDHC2_VSELECT or similar, (so SD controller
>>> driver can operate the pin via SD controller), but set SION bit so GPIO
>>> controller can read its state (activate bit 30), i.e.:
>>> MX8MP_IOMUXC_GPIO1_IO04__USDHC2_VSELECT 0x40000004
>>>
>>> Use sd-vsel-gpios property of PMIC, claim the VSELECT as GPIO in PMIC
>>> driver (this won't change pinmux, the pin would still be configured as
>>> function, but SION bit would allow you to read its state), read its
>>> state, and return the correct LDO5CTRL_H/LDO5CTRL_L value based on that.
>>
>> Good idea, I will try that.
>>
>>>
>>> If this works, then:
>>>      arm64: dts: imx8mp: Drop sd-vsel-gpios from *
>>> Linux kernel patches should not be applied.
>>
>> Yes, but we could also just revert this change once the fix suggested
>> above works and is in place. Until then the sd-vsel-gpios doesn't do
>> anything.
> 
> Please give it a go, let's see if that's the way to go.

So far it looks like this would work. I came up with these patches here
[1]. I don't know if I really should send fixes for other boards (maybe
as RFC). It looks like no one else has reported such problems on other
boards so far and I can't really tell if all are affected the same way
as our Kontron boards.

So if anyone can do some tests on their hardware that would be great.

And there is also still some work to do for applying the same changes in
Linux.

[1] https://github.com/fschrempf/u-boot/commits/pca9450-sd-vsel-fixups