drm/vkms: Compute CRC without change input data

This commit decouples pixel manipulation from compute_crc() for avoiding
any pixel change during the CRC calculation. This commits represents a
preparation work for making VKMS able to support the writeback feature.

Change in V5 (Melissa):
- Rebase and drop bitmap for alpha
Change in V4 (Emil):
- Move bitmap_clear operation and comments to get_pixel function
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200830142000.146706-3-rodrigosiqueiramelo@gmail.com

drivers/gpu/drm/vkms/vkms_composer.c

@@ -9,31 +9,41 @@
 
 #include "vkms_drv.h"
 
+static u32 get_pixel_from_buffer(int x, int y, const u8 *buffer,
+				 const struct vkms_composer *composer)
+{
+	u32 pixel;
+	int src_offset = composer->offset + (y * composer->pitch)
+				      + (x * composer->cpp);
+
+	pixel = *(u32 *)&buffer[src_offset];
+
+	return pixel;
+}
+
 /**
  * compute_crc - Compute CRC value on output frame
  *
- * @vaddr_out: address to final framebuffer
+ * @vaddr: address to final framebuffer
  * @composer: framebuffer's metadata
  *
  * returns CRC value computed using crc32 on the visible portion of
  * the final framebuffer at vaddr_out
  */
-static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
+static uint32_t compute_crc(const u8 *vaddr,
+			    const struct vkms_composer *composer)
 {
-	int i, j, src_offset;
+	int x, y;
+	u32 crc = 0, pixel = 0;
 	int x_src = composer->src.x1 >> 16;
 	int y_src = composer->src.y1 >> 16;
 	int h_src = drm_rect_height(&composer->src) >> 16;
 	int w_src = drm_rect_width(&composer->src) >> 16;
-	u32 crc = 0;
-
-	for (i = y_src; i < y_src + h_src; ++i) {
-		for (j = x_src; j < x_src + w_src; ++j) {
-			src_offset = composer->offset
-				     + (i * composer->pitch)
-				     + (j * composer->cpp);
-			crc = crc32_le(crc, vaddr_out + src_offset,
-				       sizeof(u32));
+
+	for (y = y_src; y < y_src + h_src; ++y) {
+		for (x = x_src; x < x_src + w_src; ++x) {
+			pixel = get_pixel_from_buffer(x, y, vaddr, composer);
+			crc = crc32_le(crc, (void *)&pixel, sizeof(u32));
 		}
 	}