On Thu, 20 Nov 2014, Stefan Brüns <stefan.bruens at rwth-aachen.de> wrote:
> 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;
I still feel like this should be embedded in drm_edid_block_valid
somehow. Who's going to print the bad edid now? Is it simply no longer
printed in this loop?
I'll defer to others; any better suggestions?
Jani.
> }
> }
>
> @@ -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
>
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--
Jani Nikula, Intel Open Source Technology Center
