drm/vkms: Avoid computing blending limits inside pre_mul_alpha_blend

The pre_mul_alpha_blend is dedicated to blending, so to avoid mixing
different concepts (coordinate calculation and color management), extract
the x_limit and x_dst computation outside of this helper.
It also increases the maintainability by grouping the computation related
to coordinates in the same place: the loop in `blend`.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241118-yuv-v14-6-2dbc2f1e222c@bootlin.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>

drivers/gpu/drm/vkms/vkms_composer.c

@@ -24,34 +24,30 @@ static u16 pre_mul_blend_channel(u16 src, u16 dst, u16 alpha)
 
 /**
  * pre_mul_alpha_blend - alpha blending equation
- * @frame_info: Source framebuffer's metadata
  * @stage_buffer: The line with the pixels from src_plane
  * @output_buffer: A line buffer that receives all the blends output
+ * @x_start: The start offset
+ * @pixel_count: The number of pixels to blend
  *
- * Using the information from the `frame_info`, this blends only the
- * necessary pixels from the `stage_buffer` to the `output_buffer`
- * using premultiplied blend formula.
+ * The pixels [0;@pixel_count) in stage_buffer are blended at [@x_start;@x_start+@pixel_count) in
+ * output_buffer.
  *
  * The current DRM assumption is that pixel color values have been already
  * pre-multiplied with the alpha channel values. See more
  * drm_plane_create_blend_mode_property(). Also, this formula assumes a
  * completely opaque background.
  */
-static void pre_mul_alpha_blend(struct vkms_frame_info *frame_info,
-				struct line_buffer *stage_buffer,
-				struct line_buffer *output_buffer)
+static void pre_mul_alpha_blend(const struct line_buffer *stage_buffer,
+				struct line_buffer *output_buffer, int x_start, int pixel_count)
 {
-	int x_dst = frame_info->dst.x1;
-	struct pixel_argb_u16 *out = output_buffer->pixels + x_dst;
-	struct pixel_argb_u16 *in = stage_buffer->pixels;
-	int x_limit = min_t(size_t, drm_rect_width(&frame_info->dst),
-			    stage_buffer->n_pixels);
+	struct pixel_argb_u16 *out = &output_buffer->pixels[x_start];
+	const struct pixel_argb_u16 *in = stage_buffer->pixels;
 
-	for (int x = 0; x < x_limit; x++) {
-		out[x].a = (u16)0xffff;
-		out[x].r = pre_mul_blend_channel(in[x].r, out[x].r, in[x].a);
-		out[x].g = pre_mul_blend_channel(in[x].g, out[x].g, in[x].a);
-		out[x].b = pre_mul_blend_channel(in[x].b, out[x].b, in[x].a);
+	for (int i = 0; i < pixel_count; i++) {
+		out[i].a = (u16)0xffff;
+		out[i].r = pre_mul_blend_channel(in[i].r, out[i].r, in[i].a);
+		out[i].g = pre_mul_blend_channel(in[i].g, out[i].g, in[i].a);
+		out[i].b = pre_mul_blend_channel(in[i].b, out[i].b, in[i].a);
 	}
 }
 
@@ -183,7 +179,7 @@ static void blend(struct vkms_writeback_job *wb,
 {
 	struct vkms_plane_state **plane = crtc_state->active_planes;
 	u32 n_active_planes = crtc_state->num_active_planes;
-	int y_pos;
+	int y_pos, x_dst, pixel_count;
 
 	const struct pixel_argb_u16 background_color = { .a = 0xffff };
 
@@ -201,14 +197,16 @@ static void blend(struct vkms_writeback_job *wb,
 
 		/* The active planes are composed associatively in z-order. */
 		for (size_t i = 0; i < n_active_planes; i++) {
+			x_dst = plane[i]->frame_info->dst.x1;
+			pixel_count = min_t(int, drm_rect_width(&plane[i]->frame_info->dst),
+					    (int)stage_buffer->n_pixels);
 			y_pos = get_y_pos(plane[i]->frame_info, y);
 
 			if (!check_limit(plane[i]->frame_info, y_pos))
 				continue;
 
 			vkms_compose_row(stage_buffer, plane[i], y_pos);
-			pre_mul_alpha_blend(plane[i]->frame_info, stage_buffer,
-					    output_buffer);
+			pre_mul_alpha_blend(stage_buffer, output_buffer, x_dst, pixel_count);
 		}
 
 		apply_lut(crtc_state, output_buffer);