[PATCH] drm/vkms: use bitfield op to get xrgb on compute crc

[Melissa Wen <melissa.srw@gmail.com>]

The previous memset operation was not correctly setting zero on the
alpha channel to compute the crc, and as a result, the
pipe-A-cursor-alpha-transparent subtest of the IGT test kms_cursor_crc
was crashing. To avoid errors of misinterpretation related to
endianness, this solution uses a bitfield operation to extract the RGB
values from each pixel and ignores the alpha channel as expected.

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
---
 drivers/gpu/drm/vkms/vkms_composer.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index 4af2f19480f4..8c1c005bb717 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0+
 
 #include <linux/crc32.h>
+#include <linux/bitfield.h>
 
 #include <drm/drm_atomic.h>
 #include <drm/drm_atomic_helper.h>
@@ -9,6 +10,8 @@
 
 #include "vkms_drv.h"
 
+#define XRGB_MSK GENMASK(23, 0)
+
 /**
  * compute_crc - Compute CRC value on output frame
  *
@@ -26,6 +29,7 @@ static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
 	int h_src = drm_rect_height(&composer->src) >> 16;
 	int w_src = drm_rect_width(&composer->src) >> 16;
 	u32 crc = 0;
+	u32 *pixel;
 
 	for (i = y_src; i < y_src + h_src; ++i) {
 		for (j = x_src; j < x_src + w_src; ++j) {
@@ -33,7 +37,8 @@ static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
 				     + (i * composer->pitch)
 				     + (j * composer->cpp);
 			/* XRGB format ignores Alpha channel */
-			memset(vaddr_out + src_offset + 24, 0,  8);
+			pixel = vaddr_out + src_offset;
+			*pixel = FIELD_GET(XRGB_MSK, *(u32 *)pixel);
 			crc = crc32_le(crc, vaddr_out + src_offset,
 				       sizeof(u32));
 		}
-- 
2.25.1

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