[PATCH] drm/edid: Refine HDMI VSDB detect

[Ville Syrjälä]

On Mon, Dec 13, 2021 at 12:42:22PM +0000, Lee, Shawn C wrote:
> 
> 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.

Hrm. This deserves a comment then since it clearly violates the spec.
Also a link to the bug if you have one would be nice to include here.

> 
> 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
> >

-- 
Ville Syrjälä
Intel