Re: [PATCH] soc: qcom: cmd-db: map shared memory as WT, not WB

[Maulik Shah (mkshah)]

On 3/29/2024 3:49 AM, Volodymyr Babchuk wrote:
> Hi Maulik
>
> "Maulik Shah (mkshah)" <quic_mkshah@xxxxxxxxxxx> writes:
>
> > On 3/28/2024 1:39 AM, Volodymyr Babchuk wrote:
> > > It appears that hardware does not like cacheable accesses to this
> > > region. Trying to access this shared memory region as Normal Memory
> > > leads to secure interrupt which causes an endless loop somewhere in
> > > Trust Zone.
> >
> > Linux does not write into cmd-db region. This region is write
> > protected by XPU. Making this region uncached magically solves the XPU
> > write fault
> > issue.
> >
> > Can you please include above details?
>
> Sure, I'll add this to the next version.

Thanks.

> > In downstream, we have below which resolved similar issue on qcm6490.
> >
> > cmd_db_header = memremap(rmem->base, rmem->size, MEMREMAP_WC);
> >
> > Downstream SA8155P also have MEMREMAP_WC. Can you please give it a try
> > on your device?
>
> Yes, MEMREMAP_WC works as well. This opens the question: which type is
> more correct? I have no deep understanding in QCOM internals so it is
> hard to me to answer this question.

XPU may have falsely detected clean cache eviction as "write" into the 
write protected region so using uncached flag MEMREMAP_WC may be helping 
here and is more correct in my understanding.

This can also be included in commit message.

Thanks,
Maulik