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

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 16.03.26 um 12:38 schrieb Jani Nikula:
> 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.

We only fix up the checksum to address the fact that we cannot read the 
extensions.

I guess the kernel could be fixed to work correctly with the extensions 
missing, but what about user space? Exporting an EDID without the 
extensions blocks could cause problems with user-space programs.

Best regards
Thomas


>
> BR,
> Jani.
>
>
>> Best regards
>> Thomas
>>
>> [1]
>> https://elixir.bootlin.com/linux/v6.19.8/source/drivers/gpu/drm/ast/ast_dp.c#L157
>>
>>>    
>>>    	/*
>>>
>>> ---
>>> base-commit: 5ee8dbf54602dc340d6235b1d6aa17c0f283f48c
>>> change-id: 20260313-upstream_ast_dp_edid-5fe6adf7ad36
>>>
>>> Best regards,

-- 
--
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)