[PATCH v2] drm/ast: DisplayPort edid supports 256 bytes

[Jammy Huang]

DisplayPort supports EDID up to 256 bytes (blocks 0 and 1). Update the
block check to allow these two blocks while returning 0 for any
additional extension blocks.

Furthermore, remove the manual EDID byte manipulation logic. The DRM
core (drm_edid) already handles error correction and checksum
validation.

Signed-off-by: Jammy Huang <jammy_huang@xxxxxxxxxxxxxx>
---
ASPEED DisplayPort's EDID size can be 256 bytes at most. Thus, EDID
blocks fetched can be 0 and 1.
---
Changes in v2:
Becasue drm-edid will handle invalid EDID if happen, we have 2 changes
below.
- Return 0 for the number of block more than 1.
- Remove modification of EDID
- Link to v1: https://lore.kernel.org/r/20260313-upstream_ast_dp_edid-v1-1-2a75b7c091b2@xxxxxxxxxxxxxx
---
 drivers/gpu/drm/ast/ast_dp.c | 18 ++----------------
 1 file changed, 2 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/drm/ast/ast_dp.c b/drivers/gpu/drm/ast/ast_dp.c
index 9d07dad358c..282c694218c 100644
--- a/drivers/gpu/drm/ast/ast_dp.c
+++ b/drivers/gpu/drm/ast/ast_dp.c
@@ -88,8 +88,8 @@ static int ast_astdp_read_edid_block(void *data, u8 *buf, unsigned int block, si
 	int ret = 0;
 	unsigned int i;
 
-	if (block > 0)
-		return -EIO; /* extension headers not supported */
+	if (block > 1)
+		return 0;
 
 	/*
 	 * Protect access to I/O registers from concurrent modesetting
@@ -154,20 +154,6 @@ static int ast_astdp_read_edid_block(void *data, u8 *buf, unsigned int block, si
 		ediddata[2] = ast_get_index_reg(ast, AST_IO_VGACRI, 0xda);
 		ediddata[3] = ast_get_index_reg(ast, AST_IO_VGACRI, 0xdb);
 
-		if (i == 31) {
-			/*
-			 * For 128-bytes EDID_1.3,
-			 * 1. Add the value of Bytes-126 to Bytes-127.
-			 *		The Bytes-127 is Checksum. Sum of all 128bytes should
-			 *		equal 0	(mod 256).
-			 * 2. Modify Bytes-126 to be 0.
-			 *		The Bytes-126 indicates the Number of extensions to
-			 *		follow. 0 represents noextensions.
-			 */
-			ediddata[3] = ediddata[3] + ediddata[2];
-			ediddata[2] = 0;
-		}
-
 		memcpy(buf, ediddata, min((len - i), 4));
 		buf += 4;
 	}

---
base-commit: 5ee8dbf54602dc340d6235b1d6aa17c0f283f48c
change-id: 20260313-upstream_ast_dp_edid-5fe6adf7ad36

Best regards,
-- 
Jammy Huang <jammy_huang@xxxxxxxxxxxxxx>