Re: [PATCH] drm/ast: DisplayPort edid supports 256 bytes

[Ville Syrjälä]

On Mon, Mar 16, 2026 at 01:38:22PM +0200, Jani Nikula wrote:
> On Mon, 16 Mar 2026, Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
> > Hi Jammy
> >
> > Am 13.03.26 um 11:04 schrieb Jammy Huang:
> >> DisplayPort supports edid at most 256 bytes. Thus, allow it to fetch
> >> edid block 0 and 1.
> >>
> >> Signed-off-by: Jammy Huang <jammy_huang@xxxxxxxxxxxxxx>
> >> ---
> >> ASPEED DisplayPort's EDID size can be 256 bytes at most. Thus, EDID
> >> blocks fetched can be 0 and 1.
> >> ---
> >>   drivers/gpu/drm/ast/ast_dp.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/ast/ast_dp.c b/drivers/gpu/drm/ast/ast_dp.c
> >> index 9d07dad358c..c938e1d6b1d 100644
> >> --- a/drivers/gpu/drm/ast/ast_dp.c
> >> +++ b/drivers/gpu/drm/ast/ast_dp.c
> >> @@ -88,7 +88,7 @@ static int ast_astdp_read_edid_block(void *data, u8 *buf, unsigned int block, si
> >>   	int ret = 0;
> >>   	unsigned int i;
> >>   
> >> -	if (block > 0)
> >> +	if (block > 1)
> >>   		return -EIO; /* extension headers not supported */
> >
> > But see the code at [1]. It clears the number of extensions to zero and 
> > updates the checksum accordingly. This is required to make the shortened 
> > EDID work with DRM. If you leave this as-is, it will still clear support 
> > for any extension in block 1.
> >
> > See the table 2.4 in the VESA EDID 1.4 standard for the semantics. For 
> > 1.3, if the number of blocks is >2, the first extensions is a 'block map 
> > of the extensions'. This is useless, as it's not a data extension in 
> > itself.  In 1.4, the block map is optional. That code should clear the 
> > EDID's number of extensions to 0 or 1, depending on whether there is a 
> > block map to be expected.
> 
> I think long-term the goal should be for the kernel to not modify the
> EDID, at all.

I think as a short term goal it would be much better if all EDID
mangling would be done by drm_edid.c rather than individual drivers.

-- 
Ville Syrjälä
Intel