[PATCH v5 2/3] drm/vkms: Compute CRC without change input data

[Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>]

The compute_crc() function is responsible for calculating the
framebuffer CRC value; due to the XRGB format, this function has to
ignore the alpha channel during the CRC computation. Therefore,
compute_crc() set zero to the alpha channel directly in the input
framebuffer, which is not a problem since this function receives a copy
of the original buffer. However, if we want to use this function in a
context without a buffer copy, it will change the initial value. This
patch makes compute_crc() calculate the CRC value without modifying the
input framebuffer.

Change in V4 (Emil):
- Move bitmap_clear operation and comments to get_pixel function

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
---
 drivers/gpu/drm/vkms/vkms_composer.c | 38 ++++++++++++++++++----------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index 4d8bc04bb6ee..387b0690a64a 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -9,33 +9,43 @@
 
 #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];
+	/* XRGB format ignores Alpha channel */
+	bitmap_clear((void *)&pixel, 0, 8);
+
+	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);
-			/* XRGB format ignores Alpha channel */
-			bitmap_clear(vaddr_out + src_offset, 24, 8);
-			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));
 		}
 	}
 
-- 
2.28.0