RE: [PATCH v2] drm/ast: DisplayPort edid supports 256 bytes

[Jammy Huang]

Hi Jani,

Thanks for your input. So, your suggestion is go back to return -EIO for block > 1,
and stop trying to modify the EDID.
In this way, we can handle EDID whose blocks is less than 2.

BR,
Jammy

> > Hi Thomas,
> >
> > Good catch - returning 0 without writing to buf wasn't safe. But I
> > also found that returning -EIO isn't right either: _drm_do_get_edid()
> > treats EDID_BLOCK_READ_FAIL as fatal for any block index, not just
> > block 0, so that would have discarded the whole EDID for any monitor
> > reporting more than one extension.
> >
> > In v3, for block > 1 I now zero the buffer explicitly and return 0.
> > That makes drm_edid's block checker classify it as EDID_BLOCK_ZERO
> > instead, which is non-fatal - it gets trimmed via
> > edid_filter_invalid_blocks() (extensions count reduced, checksum
> > recomputed), and blocks 0 and 1 still come through fine. This also
> > directly addresses your point about the buffer not being guaranteed
> > zeroed, since we zero it ourselves now.
> 
> There's a long-term goal to *not* modify the EDID in kernel, but rather return
> even the broken EDID extensions to userspace, if that's what the display has.
> 
> There are probably a bunch of hurdles in making that happen, but intentionally
> returning zeroed out EDID blocks is just adding another unnecessary hurdle.
> 
> 
> BR,
> Jani.
> 
> 
> --
> Jani Nikula, Intel