imx6ull: random crashes when setting REFTOP_SELBIASOFF

[Heiko Schocher]

Hello Fabio, Peng,

I have here an imx6ull based board from DH electronics [1]
which has the IMX_THERMAL driver enabled in U-Boot and Linux. We
see on Linux boot random crashes (random means not on every boot,
also not even the same crash dump/reason in linux). My first
suggestion was instable RMA, but RAM seems stable.

After some investigation it turned out, that setting the
REFTOP_SELBIASOFF bit in the linux thermal driver seems the problem.

I saw that the kernel driver does not check first if REFTOP_VBGUP
is set and added this, but this did not helped.

U-Boot does this check here:
  https://gitlab.denx.de/u-boot/u-boot/blob/master/arch/arm/mach-imx/mx6/soc.c#L342

More investigation into this problem and we found how U-Boot handles
this bit:

U-Boot: sets this bit in  arch_cpu_init()
U-Boot: sets this bit in IMX_THERMAL driver
U-Boot: clears this bit in IMX_THERMAL driver


If we now remove the clearing of this bit in U-Boot in the thermal
driver, we also see no crashes anymore in Linux (more than 12000
power on/off cycles)

My question is now,

- do you now more about this bit as in the RM is described?
   Or know somebody we can ask?

- What effect has this bit? May it leads in more power consumption?

- Is it valid to not clear it in U-Boot?

Thanks in advance!

bye,
Heiko
[1] https://github.com/dh-electronics/u-boot-imx6qdl/blob/dev/2018.05_dhcom/include/configs/dh_imx6ull.h
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