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

[Thomas Zimmermann <tzimmermann@suse.de>]

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.

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)