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SubjectRe: framebuffer corruption due to overlapping stp instructions on arm64


On Wed, 8 Aug 2018, Catalin Marinas wrote:

> On Fri, Aug 03, 2018 at 01:09:02PM -0400, Mikulas Patocka wrote:
> > while (1) {
> > start = (unsigned)random() % (LEN + 1);
> > end = (unsigned)random() % (LEN + 1);
> > if (start > end)
> > continue;
> > for (i = start; i < end; i++)
> > data[i] = val++;
> > memcpy(map + start, data + start, end - start);
> > if (memcmp(map, data, LEN)) {
>
> It may be worth trying to do a memcmp(map+start, data+start, end-start)
> here to see whether the hazard logic fails when the writes are unaligned
> but the reads are not.
>
> This problem may as well appear if you do byte writes and read longs
> back (and I consider this a hardware problem on this specific board).

I triad to insert usleep(10000) between the memcpy and memcmp, but the
same corruption occurs. So, it can't be read-after-write hazard. It is
caused by the improper handling of hazard between the overlapping writes
inside memcpy.

Mikulas

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