[PATCH 2/2] drm/edid: Tighten checksum conditions for CEA blocks

[Stefan Brüns]

4a638b4e38234233f5c7e6705662fbc0b58d80c2 disabled the checksumming
for CEA blocks. If only the checksum is wrong, reading twice should
result in identical data, whereas a bad transfer will most likely
corrupt diffent bytes.

Signed-off-by: Stefan Brüns <stefan.bruens at rwth-aachen.de>
---
 drivers/gpu/drm/drm_edid.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 505960e..9b6b65e 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -1200,6 +1200,7 @@ drm_do_get_edid(struct drm_connector *connector, struct i2c_adapter *adapter)
 {
 	int i, j = 0, valid_extensions = 0;
 	u8 *block, *new;
+	u8 *saved_block = NULL;
 	bool print_bad_edid = !connector->bad_edid_counter || (drm_debug & DRM_UT_KMS);
 
 	if ((block = kmalloc(EDID_LENGTH, GFP_KERNEL)) == NULL)
@@ -1234,15 +1235,29 @@ drm_do_get_edid(struct drm_connector *connector, struct i2c_adapter *adapter)
 	block = new;
 
 	for (j = 1; j <= block[0x7e]; j++) {
+		u8 *ext_block = block + (valid_extensions + 1) * EDID_LENGTH;
+		u8 csum, last_csum = 0;
 		for (i = 0; i < 4; i++) {
-			if (drm_do_probe_ddc_edid(adapter,
-				  block + (valid_extensions + 1) * EDID_LENGTH,
-				  j, EDID_LENGTH))
+			if (drm_do_probe_ddc_edid(adapter, ext_block, j, EDID_LENGTH))
 				goto out;
-			if (drm_edid_block_valid(block + (valid_extensions + 1) * EDID_LENGTH, j, print_bad_edid)) {
+			if ((csum = drm_edid_block_checksum(ext_block)) == 0) {
 				valid_extensions++;
 				break;
+			} else if ((ext_block[0] == CEA_EXT) && (csum == last_csum)) {
+			/*
+			 * Some switches mangle CEA contents without fixing the checksum.
+			 * Accept CEA blocks when two reads return identical data.
+			 */
+				if (!saved_block)
+					saved_block = kmalloc(EDID_LENGTH, GFP_KERNEL);
+				if (saved_block && !memcmp(ext_block, saved_block, EDID_LENGTH)) {
+					valid_extensions++;
+					break;
+				}
+				if (saved_block)
+					memcpy(saved_block, ext_block, EDID_LENGTH);
 			}
+			last_csum = csum;
 		}
 
 		if (i == 4 && print_bad_edid) {
@@ -1263,6 +1278,7 @@ drm_do_get_edid(struct drm_connector *connector, struct i2c_adapter *adapter)
 		block = new;
 	}
 
+	kfree(saved_block);
 	return block;
 
 carp:
@@ -1273,6 +1289,7 @@ carp:
 	connector->bad_edid_counter++;
 
 out:
+	kfree(saved_block);
 	kfree(block);
 	return NULL;
 }
-- 
1.8.4.5