[Intel-gfx] [PATCH] drm/i915: Unconditionally flush residual writes before batches

[Chris Wilson]

During batch buffer emission we flag that the next request should
generate a flush. The intention is that we queue a request following the
batch buffer in order to provide the breadcrumb, and so always flush all
caches after every batch, killing the flushing list. However, X
carefully schedules a signal to arrive just as we are checking whether
the ring has enough space to enqueue the flush and so we abort that
addition with an ERESTARTSYS. The next batch then clears the write_domain,
and we end up with an object with no write_domain on the ring's
gpu_write_list. Havoc WARNs and BUGs ensue.

The gpu_write_list is doomed as it is merely a remnant  of the flushing
list, so the easiest fix in the meantime is to clear all residual
members from the gpu_write_list prior to updating the write_domain on
the next batch.

Fixes regression from

commit cc889e0f6ce6a63c62db17d702ecfed86d58083f
Author: Daniel Vetter <daniel.vetter at ffwll.ch>
Date:   Wed Jun 13 20:45:19 2012 +0200

    drm/i915: disable flushing_list/gpu_write_lis

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc:  Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 88e2e11..4c521df 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -885,8 +885,9 @@ i915_gem_execbuffer_move_to_gpu(struct intel_ring_buffer *ring,
 			return ret;
 	}
 
-	/* Unconditionally invalidate gpu caches. */
-	ret = i915_gem_flush_ring(ring, I915_GEM_GPU_DOMAINS, 0);
+	/* Unconditionally invalidate gpu caches and flush residual writes. */
+	ret = i915_gem_flush_ring(ring,
+				  I915_GEM_GPU_DOMAINS, I915_GEM_GPU_DOMAINS);
 	if (ret)
 		return ret;
 
-- 
1.7.10.4