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Re: framebuffer corruption due to overlapping stp instructions on arm64

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?

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

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.

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