drm/i915: Split obj->cache_coherent to track r/w

Another month, another story in the cache coherency saga. This time, we
come to the realisation that i915_gem_object_is_coherent() has been
reporting whether we can read from the target without requiring a cache
invalidate; but we were using it in places for testing whether we could
write into the object without requiring a cache flush. So split the
tracking into two, one to decide before reads, one after writes.

See commit e27ab73d17ef ("drm/i915: Mark CPU cache as dirty on every
transition for CPU writes") for the previous entry in this saga.

v2: Be verbose
v3: Remove unused function (i915_gem_object_is_coherent)
v4: Fix inverted coherency check prior to execbuf (from v2)
v5: Add comment for nasty code where we are optimising on gcc's behalf.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101109
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101555
Testcase: igt/kms_mmap_write_crc
Testcase: igt/kms_pwrite_crc
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Dongwon Kim <dongwon.kim@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Tested-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170811111116.10373-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 5fa4476..99520b2 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1842,7 +1842,19 @@ static int eb_move_to_gpu(struct i915_execbuffer *eb)
 			eb->request->capture_list = capture;
 		}
 
-		if (unlikely(obj->cache_dirty && !obj->cache_coherent)) {
+		/*
+		 * If the GPU is not _reading_ through the CPU cache, we need
+		 * to make sure that any writes (both previous GPU writes from
+		 * before a change in snooping levels and normal CPU writes)
+		 * caught in that cache are flushed to main memory.
+		 *
+		 * We want to say
+		 *   obj->cache_dirty &&
+		 *   !(obj->cache_coherent & I915_BO_CACHE_COHERENT_FOR_READ)
+		 * but gcc's optimiser doesn't handle that as well and emits
+		 * two jumps instead of one. Maybe one day...
+		 */
+		if (unlikely(obj->cache_dirty & ~obj->cache_coherent)) {
 			if (i915_gem_clflush_object(obj, 0))
 				entry->flags &= ~EXEC_OBJECT_ASYNC;
 		}