[PATCH 4.4 088/138] drm: Restore double clflush on the last partial cacheline

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Chris Wilson <chris at chris-wilson.co.uk>

commit 396f5d62d1a5fd99421855a08ffdef8edb43c76e upstream.

This effectively reverts

commit afcd950cafea6e27b739fe7772cbbeed37d05b8b
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Wed Jun 10 15:58:01 2015 +0100

    drm: Avoid the double clflush on the last cache line in drm_clflush_virt_range()

as we have observed issues with serialisation of the clflush operations
on Baytrail+ Atoms with partial updates. Applying the double flush on the
last cacheline forces that clflush to be ordered with respect to the
previous clflush, and the mfence then protects against prefetches crossing
the clflush boundary.

The same issue can be demonstrated in userspace with igt/gem_exec_flush.

Fixes: afcd950cafea6 (drm: Avoid the double clflush on the last cache...)
Testcase: igt/gem_concurrent_blit
Testcase: igt/gem_partial_pread_pwrite
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92845
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: dri-devel at lists.freedesktop.org
Cc: Akash Goel <akash.goel at intel.com>
Cc: Imre Deak <imre.deak at intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Jason Ekstrand <jason.ekstrand at intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala at intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1467880930-23082-6-git-send-email-chris@chris-wilson.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>

---
 drivers/gpu/drm/drm_cache.c |    1 +
 1 file changed, 1 insertion(+)

--- a/drivers/gpu/drm/drm_cache.c
+++ b/drivers/gpu/drm/drm_cache.c
@@ -136,6 +136,7 @@ drm_clflush_virt_range(void *addr, unsig
 		mb();
 		for (; addr < end; addr += size)
 			clflushopt(addr);
+		clflushopt(end - 1); /* force serialisation */
 		mb();
 		return;
 	}