[PATCH 3/3] drm/edid: Tighten checksum conditions for CEA blocks

[Stefan Brüns]

Checksumming was disabled for CEA blocks by

commit 4a638b4e38234233f5c7e6705662fbc0b58d80c2
Author: Adam Jackson <ajax at redhat.com>
Date:   Tue May 25 16:33:09 2010 -0400

    drm/edid: Allow non-fatal checksum errors in CEA blocks

If only the checksum is wrong, reading twice should result in identical
data, whereas a bad transfer will most likely corrupt different bytes.
Comparing checksums is not sufficient, as there is a considerable chance
of two bad transfers having the same checksum.

Signed-off-by: Stefan Brüns <stefan.bruens at rwth-aachen.de>
---
 drivers/gpu/drm/drm_edid.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 3a10f3f..55963d5 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -1203,6 +1203,7 @@ 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)
@@ -1233,12 +1234,27 @@ drm_do_get_edid(struct drm_connector *connector, struct i2c_adapter *adapter)
 
 	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, ext_block, j, EDID_LENGTH))
 				goto out;
-			if (drm_edid_block_valid(ext_block, j, print_bad_edid)) {
+			csum = drm_edid_block_csum(ext_block);
+			if (!csum) {
 				valid_extensions++;
 				break;
+			} else if (ext_block[0] == CEA_EXT) {
+			/*
+			 * Some switches mangle CEA contents without fixing the checksum.
+			 * Accept CEA blocks when two reads return identical data.
+			 */
+				if (saved_block && csum == last_csum &&
+				    !memcmp(ext_block, saved_block, EDID_LENGTH)) {
+					valid_extensions++;
+					break;
+				}
+				kfree(saved_block);
+				saved_block = kmemdup(ext_block, EDID_LENGTH, GFP_KERNEL);
+				last_csum = csum;
 			}
 		}
 
@@ -1249,6 +1265,9 @@ drm_do_get_edid(struct drm_connector *connector, struct i2c_adapter *adapter)
 
 			connector->bad_edid_counter++;
 		}
+
+		kfree(saved_block);
+		saved_block = NULL;
 	}
 
 	if (valid_extensions != block[0x7e]) {
@@ -1270,6 +1289,7 @@ carp:
 	connector->bad_edid_counter++;
 
 out:
+	kfree(saved_block);
 	kfree(block);
 	return NULL;
 }
-- 
2.1.2