drm/vc4: Add a proper short-circut path for legacy cursor updates.

Previously, on every modeset we would allocate new display list
memory, recompute changed planes, write all of them to the new memory,
and pointed scanout at the new list (which will latch approximately at
the next line of scanout).  We let
drm_atomic_helper_wait_for_vblanks() decide whether we needed to wait
for a vblank after a modeset before cleaning up the old state and
letting the next modeset proceed, and on legacy cursor updates we
wouldn't wait.  If you moved the cursor fast enough, we could
potentially wrap around the display list memory area and overwrite the
existing display list while it was still being scanned out, resulting
in the HVS scanning out garbage or just halting.

Instead of making cursor updates wait for scanout to move to the new
display list area (which introduces significant cursor lag in X), we
just rewrite our current display list.
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/vc4/vc4_kms.c

@@ -49,6 +49,15 @@ vc4_atomic_complete_commit(struct vc4_commit *c)
 
 	drm_atomic_helper_commit_modeset_enables(dev, state);
 
+	/* Make sure that drm_atomic_helper_wait_for_vblanks()
+	 * actually waits for vblank.  If we're doing a full atomic
+	 * modeset (as opposed to a vc4_update_plane() short circuit),
+	 * then we need to wait for scanout to be done with our
+	 * display lists before we free it and potentially reallocate
+	 * and overwrite the dlist memory with a new modeset.
+	 */
+	state->legacy_cursor_update = false;
+
 	drm_atomic_helper_wait_for_vblanks(dev, state);
 
 	drm_atomic_helper_cleanup_planes(dev, state);

drivers/gpu/drm/vc4/vc4_plane.c

@@ -33,8 +33,12 @@ struct vc4_plane_state {
 	u32 dlist_size; /* Number of dwords allocated for the display list */
 	u32 dlist_count; /* Number of used dwords in the display list. */
 
-	/* Offset in the dlist to pointer word 0. */
-	u32 pw0_offset;
+	/* Offset in the dlist to various words, for pageflip or
+	 * cursor updates.
+	 */
+	u32 pos0_offset;
+	u32 pos2_offset;
+	u32 ptr0_offset;
 
 	/* Offset where the plane's dlist was last stored in the
 	 * hardware at vc4_crtc_atomic_flush() time.
@@ -223,6 +227,7 @@ static int vc4_plane_mode_set(struct drm_plane *plane,
 			SCALER_CTL0_UNITY);
 
 	/* Position Word 0: Image Positions and Alpha Value */
+	vc4_state->pos0_offset = vc4_state->dlist_count;
 	vc4_dlist_write(vc4_state,
 			VC4_SET_FIELD(0xff, SCALER_POS0_FIXED_ALPHA) |
 			VC4_SET_FIELD(vc4_state->crtc_x, SCALER_POS0_START_X) |
@@ -233,6 +238,7 @@ static int vc4_plane_mode_set(struct drm_plane *plane,
 	 */
 
 	/* Position Word 2: Source Image Size, Alpha Mode */
+	vc4_state->pos2_offset = vc4_state->dlist_count;
 	vc4_dlist_write(vc4_state,
 			VC4_SET_FIELD(format->has_alpha ?
 				      SCALER_POS2_ALPHA_MODE_PIPELINE :
@@ -244,9 +250,8 @@ static int vc4_plane_mode_set(struct drm_plane *plane,
 	/* Position Word 3: Context.  Written by the HVS. */
 	vc4_dlist_write(vc4_state, 0xc0c0c0c0);
 
-	vc4_state->pw0_offset = vc4_state->dlist_count;
-
 	/* Pointer Word 0: RGB / Y Pointer */
+	vc4_state->ptr0_offset = vc4_state->dlist_count;
 	vc4_dlist_write(vc4_state, bo->paddr + vc4_state->offset);
 
 	/* Pointer Context Word 0: Written by the HVS */
@@ -332,13 +337,13 @@ void vc4_plane_async_set_fb(struct drm_plane *plane, struct drm_framebuffer *fb)
 	 * scanout will start from this address as soon as the FIFO
 	 * needs to refill with pixels.
 	 */
-	writel(addr, &vc4_state->hw_dlist[vc4_state->pw0_offset]);
+	writel(addr, &vc4_state->hw_dlist[vc4_state->ptr0_offset]);
 
 	/* Also update the CPU-side dlist copy, so that any later
 	 * atomic updates that don't do a new modeset on our plane
 	 * also use our updated address.
 	 */
-	vc4_state->dlist[vc4_state->pw0_offset] = addr;
+	vc4_state->dlist[vc4_state->ptr0_offset] = addr;
 }
 
 static const struct drm_plane_helper_funcs vc4_plane_helper_funcs = {
@@ -354,8 +359,83 @@ static void vc4_plane_destroy(struct drm_plane *plane)
 	drm_plane_cleanup(plane);
 }
 
+/* Implements immediate (non-vblank-synced) updates of the cursor
+ * position, or falls back to the atomic helper otherwise.
+ */
+static int
+vc4_update_plane(struct drm_plane *plane,
+		 struct drm_crtc *crtc,
+		 struct drm_framebuffer *fb,
+		 int crtc_x, int crtc_y,
+		 unsigned int crtc_w, unsigned int crtc_h,
+		 uint32_t src_x, uint32_t src_y,
+		 uint32_t src_w, uint32_t src_h)
+{
+	struct drm_plane_state *plane_state;
+	struct vc4_plane_state *vc4_state;
+
+	if (plane != crtc->cursor)
+		goto out;
+
+	plane_state = plane->state;
+	vc4_state = to_vc4_plane_state(plane_state);
+
+	if (!plane_state)
+		goto out;
+
+	/* If we're changing the cursor contents, do that in the
+	 * normal vblank-synced atomic path.
+	 */
+	if (fb != plane_state->fb)
+		goto out;
+
+	/* No configuring new scaling in the fast path. */
+	if (crtc_w != plane_state->crtc_w ||
+	    crtc_h != plane_state->crtc_h ||
+	    src_w != plane_state->src_w ||
+	    src_h != plane_state->src_h) {
+		goto out;
+	}
+
+	/* Set the cursor's position on the screen.  This is the
+	 * expected change from the drm_mode_cursor_universal()
+	 * helper.
+	 */
+	plane_state->crtc_x = crtc_x;
+	plane_state->crtc_y = crtc_y;
+
+	/* Allow changing the start position within the cursor BO, if
+	 * that matters.
+	 */
+	plane_state->src_x = src_x;
+	plane_state->src_y = src_y;
+
+	/* Update the display list based on the new crtc_x/y. */
+	vc4_plane_atomic_check(plane, plane_state);
+
+	/* Note that we can't just call vc4_plane_write_dlist()
+	 * because that would smash the context data that the HVS is
+	 * currently using.
+	 */
+	writel(vc4_state->dlist[vc4_state->pos0_offset],
+	       &vc4_state->hw_dlist[vc4_state->pos0_offset]);
+	writel(vc4_state->dlist[vc4_state->pos2_offset],
+	       &vc4_state->hw_dlist[vc4_state->pos2_offset]);
+	writel(vc4_state->dlist[vc4_state->ptr0_offset],
+	       &vc4_state->hw_dlist[vc4_state->ptr0_offset]);
+
+	return 0;
+
+out:
+	return drm_atomic_helper_update_plane(plane, crtc, fb,
+					      crtc_x, crtc_y,
+					      crtc_w, crtc_h,
+					      src_x, src_y,
+					      src_w, src_h);
+}
+
 static const struct drm_plane_funcs vc4_plane_funcs = {
-	.update_plane = drm_atomic_helper_update_plane,
+	.update_plane = vc4_update_plane,
 	.disable_plane = drm_atomic_helper_disable_plane,
 	.destroy = vc4_plane_destroy,
 	.set_property = NULL,