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Chris Wilson authored
We don't expect every machine to be able to pass the WC/GTT coherency check, see kernel commit 3b5724d702ef24ee41ca008a1fab1cf94f3d31b5 Author: Chris Wilson <chris@chris-wilson.co.uk> Date: Thu Aug 18 17:16:49 2016 +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. so split out the interleave coherency check from the basic interopability check. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102577 Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
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