[Intel-gfx] [PATCH 1/2] drm/edid: If no preferred mode is found assume the first mode is preferred

[Adam Jackson]

On Wed, 2019-02-27 at 19:14 +0200, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Some monitors apparently forget to mark any mode as preferred in the
> EDID. In this particular case we have a very generic looking ID
> "PNP Model 0 Serial Number 4" / "LVDS 800x600" so a specific quirk
> doesn't seem particularly wise. Also the quirk we have
> (EDID_QUIRK_FIRST_DETAILED_PREFERRED) is actually defunct so we'd
> have to fix it first.
>
> As a generic fallback let's just mark the first probed mode (which
> should be the first detailed mode, assuming there are any) as
> preferred.

What problem is this trying to fix? Userspace (and drm for that matter)
is typically going to pick the first mode in the list anyway if there's
none marked as preferred. Not having any preferred modes was pretty
common on CRTs IIRC.

The other major case I've seen of a monitor with no preferred mode are
the early dual-link DVI displays without internal scalers (Apple
Cinema, Dell 3007WFP, etc). You end up with 1280x800 first in the list
since 2560x1600 doesn't fit in a single DVI link. It might be nice if
such monitors decided their preferred mode based on the link
capabilities; if you know it's a dual-link capable port, you'd probably
prefer 2560x1600.

Does it make more sense to run the "infer a preferred mode" logic after
we've done mode validation for the output?

- ajax