[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