tree 5fa3688f162b6aabd1413702b9bebdcc45d2530e
parent a314d5cb4ac3722b9a673656e2499f4d92ee5e6f
author Chris Wilson <chris@chris-wilson.co.uk> 1471537009 +0100
committer Chris Wilson <chris@chris-wilson.co.uk> 1471556205 +0100

drm/i915: Wait for writes through the GTT to land before reading back

If we quickly switch from writing through the GTT to a read of the
physical page directly with the CPU (e.g. performing relocations through
the GTT and then running the command parser), we can observe that the
writes are not visible to the CPU. It is not a coherency problem, as
extensive investigations with clflush have demonstrated, but a mere
timing issue - we have to wait for the GTT to complete it's write before
we start our read from the CPU.

The issue can be illustrated in userspace with:

	gtt = gem_mmap__gtt(fd, handle, 0, OBJECT_SIZE, PROT_READ | PROT_WRITE);
	cpu = gem_mmap__cpu(fd, handle, 0, OBJECT_SIZE, PROT_READ | PROT_WRITE);
	gem_set_domain(fd, handle, I915_GEM_DOMAIN_GTT, I915_GEM_DOMAIN_GTT);

	for (i = 0; i < OBJECT_SIZE / 64; i++) {
		int x = 16*i + (i%16);
		gtt[x] = i;
		clflush(&cpu[x], sizeof(cpu[x]));
		assert(cpu[x] == i);
	}

Experimenting with that shows that this behaviour is indeed limited to
recent Atom-class hardware.

Testcase: igt/gem_exec_flush/basic-batch-default-cmd #byt
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-10-chris@chris-wilson.co.uk