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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Mar 17, 2026 at 08:41:35AM +0100, Thomas Zimmermann wrote:
> Hi
> 
> Am 17.03.26 um 08:09 schrieb Ville Syrjälä:
> > On Mon, Mar 16, 2026 at 01:38:22PM +0200, Jani Nikula wrote:
> >> On Mon, 16 Mar 2026, Thomas Zimmermann <tzimmermann@suse.de> 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@aspeedtech.com>
> >>>> ---
> >>>> 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.
> 
> We don't fix the EDID here, but work around a hardware limitation. I 
> guess we could fix this automatically near edid_block_read() [1] if the 
> driver clearly communicates this issue.  The read loop could then fix 
> the header's checksum by itself.

edid_filter_invalid_blocks() already has the code to update
the extension block count and fix up the checksum.

So seems to me all we'd need is some way for the driver to
indicate it simply cannot read the requested block (based on
which edid_read_block() would return some other value than
EDID_BLOCK_READ_FAIL), and then the fixup happens naturally.

Hmm. What happens if the driver returns "success" for the
read, but just leaves the entire block zeroed? Looks to me
like we should then end up with status==EDID_BLOCK_ZERO
and the block will get treated as invalid and filtered out.

> Best regards
> Thomas
> 
> [1] 
> https://elixir.bootlin.com/linux/v6.19.8/source/drivers/gpu/drm/drm_edid.c#L2424
> 
> >
> 
> -- 
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
> GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)
> 

-- 
Ville Syrjälä
Intel