framebuffer corruption due to overlapping stp instructions on arm64

[Mikulas Patocka]

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