[PATCH] drm/edid: Refine HDMI VSDB detect

[Lee, Shawn C]

On Monday, December 13, 2021 8:31 PM, Ville Syrjälä wrote:
>On Sun, Dec 12, 2021 at 11:33:31PM +0800, Lee Shawn C wrote:
>> According to CEA-861-F chapter 7.5.4. It says "The VSDB shall contain 
>> the
>> 3 bytes of the IEEE OUI as well as any additional payload bytes needed."
>> Now DRM driver check HDMI OUI but VSDB payload size at least five bytes.
>> That may caused some HDMI monitors' audio feature can't be enabled.
>> Because of they only have three bytes payload (OUI only) in VSDB.
>
>HDMI 1.4a says
>"Sinks shall contain an HDMI VSDB minimally containing a 2-byte Source  Physical Address field following the 24-bit identifier. ...
> The minimum value of N (length) is 5 and the maximum value of N is 31."
>
>Do you actually have an EDID that violates that?
>

Yes! User report when connect to HDMI port on Acer V226HQL. Audio is not working.
But windows system did not have the same problem. We found its VSDB just have 3 bytes
payload (OUI). Then we share this patch to user then they report audio works properly
with this patch.

Best regards,
Shawn

>> 
>> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
>> Cc: Adam Jackson <ajax at redhat.com>
>> Cc: Dave Airlie <airlied at redhat.com>
>> Signed-off-by: Lee Shawn C <shawn.c.lee at intel.com>
>> ---
>>  drivers/gpu/drm/drm_edid.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c 
>> index 12893e7be89b..5aa4a6bf4a13 100644
>> --- a/drivers/gpu/drm/drm_edid.c
>> +++ b/drivers/gpu/drm/drm_edid.c
>> @@ -4205,7 +4205,7 @@ static bool cea_db_is_hdmi_vsdb(const u8 *db)
>>  	if (cea_db_tag(db) != VENDOR_BLOCK)
>>  		return false;
>>  
>> -	if (cea_db_payload_len(db) < 5)
>> +	if (cea_db_payload_len(db) < 3)
>>  		return false;
>>  
>>  	return oui(db[3], db[2], db[1]) == HDMI_IEEE_OUI;
>> --
>> 2.31.1
>
>--
>Ville Syrjälä
>Intel
>