drm/amd/display: Fix DisplayID not-found handling in parse_edid_displayid_vrr()

[ Upstream commit 2323b01 ]

parse_edid_displayid_vrr() searches the EDID extension blocks for a
DisplayID extension before parsing the dynamic video timing range.

The code previously checked whether edid_ext was NULL after the search
loop. However, edid_ext is assigned during each iteration of the loop,
so it will never be NULL once the loop has executed. If no DisplayID
extension is found, edid_ext ends up pointing to the last extension
block, and the NULL check does not correctly detect the failure case.

Instead, check whether the loop completed without finding a matching
DisplayID block by testing "i == edid->extensions". This ensures the
function exits early when no DisplayID extension is present and avoids
parsing an unrelated EDID extension block.

Also simplify the EDID validation check using "!edid ||
!edid->extensions".

Fixes the below:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:13079 parse_edid_displayid_vrr() warn: variable dereferenced before check 'edid_ext' (see line 13075)

Fixes: a638b83 ("drm/amd/display: Fix refresh rate range for some panel")
Cc: Roman Li <roman.li@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Jerry Zuo <jerry.zuo@amd.com>
Cc: Sun peng Li <sunpeng.li@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 91c7e63)
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: srishanm

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -13101,7 +13101,7 @@ static void parse_edid_displayid_vrr(struct drm_connector *connector,
 	u16 min_vfreq;
 	u16 max_vfreq;
 
-	if (edid == NULL || edid->extensions == 0)
+	if (!edid || !edid->extensions)
 		return;
 
 	/* Find DisplayID extension */
@@ -13111,7 +13111,7 @@ static void parse_edid_displayid_vrr(struct drm_connector *connector,
 			break;
 	}
 
-	if (edid_ext == NULL)
+	if (i == edid->extensions)
 		return;
 
 	while (j < EDID_LENGTH) {