drm/vc4: ->x_scaling[1] should never be set to VC4_SCALING_NONE

For the YUV conversion to work properly, ->x_scaling[1] should never
be set to VC4_SCALING_NONE, but vc4_get_scaling_mode() might return
VC4_SCALING_NONE if the horizontal scaling ratio exactly matches the
horizontal subsampling factor. Add a test to turn VC4_SCALING_NONE
into VC4_SCALING_PPF when that happens.

The old ->x_scaling[0] adjustment is dropped as I couldn't find any
mention to this constraint in the spec and it's proven to be
unnecessary (I tested various multi-planar YUV formats with scaling
disabled, and all of them worked fine without this adjustment).

Fixes: fc04023f ("drm/vc4: Add support for YUV planes.")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181109102633.32603-1-boris.brezillon@bootlin.com

drivers/gpu/drm/vc4/vc4_plane.c

@@ -341,12 +341,14 @@ static int vc4_plane_setup_clipping_and_scaling(struct drm_plane_state *state)
 			vc4_get_scaling_mode(vc4_state->src_h[1],
 					     vc4_state->crtc_h);
 
-		/* YUV conversion requires that horizontal scaling be enabled,
-		 * even on a plane that's otherwise 1:1. Looks like only PPF
-		 * works in that case, so let's pick that one.
+		/* YUV conversion requires that horizontal scaling be enabled
+		 * on the UV plane even if vc4_get_scaling_mode() returned
+		 * VC4_SCALING_NONE (which can happen when the down-scaling
+		 * ratio is 0.5). Let's force it to VC4_SCALING_PPF in this
+		 * case.
 		 */
-		if (vc4_state->is_unity)
-			vc4_state->x_scaling[0] = VC4_SCALING_PPF;
+		if (vc4_state->x_scaling[1] == VC4_SCALING_NONE)
+			vc4_state->x_scaling[1] = VC4_SCALING_PPF;
 	} else {
 		vc4_state->is_yuv = false;
 		vc4_state->x_scaling[1] = VC4_SCALING_NONE;