drm/i915: Use atomics to manipulate obj->frontbuffer_bits

The individual bits inside obj->frontbuffer_bits are protected by each
plane->mutex, but the whole bitfield may be accessed by multiple KMS
operations simultaneously and so the RMW need to be under atomics.
However, for updating the single field we do not need to mandate that it
be under the struct_mutex, one more step towards its removal as the de
facto BKL.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-21-git-send-email-chris@chris-wilson.co.uk

1 files changed, 14 insertions, 7 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 68110b9..03eb094 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4040,7 +4040,7 @@ void i915_gem_free_object(struct drm_gem_object *gem_obj)
 	if (obj->stolen)
 		i915_gem_object_unpin_pages(obj);
 
-	WARN_ON(obj->frontbuffer_bits);
+	WARN_ON(atomic_read(&obj->frontbuffer_bits));
 
 	if (obj->pages && obj->madv == I915_MADV_WILLNEED &&
 	    dev_priv->quirks & QUIRK_PIN_SWIZZLED_PAGES &&
@@ -4557,16 +4557,23 @@ void i915_gem_track_fb(struct drm_i915_gem_object *old,
 		       struct drm_i915_gem_object *new,
 		       unsigned frontbuffer_bits)
 {
+	/* Control of individual bits within the mask are guarded by
+	 * the owning plane->mutex, i.e. we can never see concurrent
+	 * manipulation of individual bits. But since the bitfield as a whole
+	 * is updated using RMW, we need to use atomics in order to update
+	 * the bits.
+	 */
+	BUILD_BUG_ON(INTEL_FRONTBUFFER_BITS_PER_PIPE * I915_MAX_PIPES >
+		     sizeof(atomic_t) * BITS_PER_BYTE);
+
 	if (old) {
-		WARN_ON(!mutex_is_locked(&old->base.dev->struct_mutex));
-		WARN_ON(!(old->frontbuffer_bits & frontbuffer_bits));
-		old->frontbuffer_bits &= ~frontbuffer_bits;
+		WARN_ON(!(atomic_read(&old->frontbuffer_bits) & frontbuffer_bits));
+		atomic_andnot(frontbuffer_bits, &old->frontbuffer_bits);
 	}
 
 	if (new) {
-		WARN_ON(!mutex_is_locked(&new->base.dev->struct_mutex));
-		WARN_ON(new->frontbuffer_bits & frontbuffer_bits);
-		new->frontbuffer_bits |= frontbuffer_bits;
+		WARN_ON(atomic_read(&new->frontbuffer_bits) & frontbuffer_bits);
+		atomic_or(frontbuffer_bits, &new->frontbuffer_bits);
 	}
 }