Re: framebuffer corruption due to overlapping stp instructions on arm64

[Sinan Kaya <okaya at kernel dot org>]

On 8/3/2018 1:09 PM, Mikulas Patocka wrote:
> > > Most accelerated graphics drivers rely heavily on the ability to map
> > > the VRAM normal-non-cacheable (ioremap_wc, basically), and treat it as
> > > ordinary memory.
> > Yeah, I'd expect framebuffers to be mapped as normal NC. That should be
> > fine for prefetchable BARs, no?
> >
> > Will
> So - why does it corrupt data then? I've created this program that
> reproduces the data corruption quicky. If I run it on /dev/fb0, I get an
> instant failure. Sometimes a few bytes are not written, sometimes a few
> bytes are written with a value that should be 16 bytes apart.
>
> I tried to run it on system RAM mapped with the NC attribute and I didn't
> get any corruption - that suggests the the bug may be in the PCIE
> subsystem.

Note that normal-NC gives you write combining whereas device
nGnRE doesn't have any write-combining support.
normal-NC is typically mapped to prefetchable BAR space where
write-combining is welcome.

It could be an issue on the SOC itself too. I suggest you contact your
board vendor.