[Intel-gfx] [PATCH 04/21] drm/i915: Only use VBT panel mode on eDP if no EDID is found

[Jesse Barnes]

On Fri, 30 Sep 2011 10:58:26 -0700
Keith Packard <keithp at keithp.com> wrote:

> On Fri, 30 Sep 2011 18:32:35 +0200, Daniel Vetter <daniel at ffwll.ch> wrote:
> 
> > Ok, this is way over my head, just checked whether the patch does what it
> > claims to - nice exercise in reading dp modeset code ;-)
> 
> Yeah, it's purely heuristic -- the VBT contains a mode which was
> originally for the LVDS. It's unclear whether it should ever be applied
> to an eDP panel, but absent other information, it seems like we should
> at least consider it.
> 
> Many LVDS panels don't bother to include an EDID rom, or the vendor
> didn't bother to hook up the DDC wire; presumably it's cheaper for them
> to stick more data in the VBIOS than add hardware.  However, there are
> some LVDS panels with EDID roms which contain *incorrect* mode data for
> the panel (amazing, I know), and so the driver prefers to use the VBT
> data when both are present.
> 
> eDP, on the other hand, doesn't require any additional wiring (at least)
> to connect up the DDC channel, and eDP panels are required to provide
> EDID data. So far, in my (very small) sample set, I've got one machine
> which provides accurate VBT *and* EDID data (an HP 2540p) and one
> machine which provides inaccurate VBT data but accurate EDID data (a
> MacBook air). So, I just decided to prefer the EDID data.
> 
> One option would be to extract the current mode from the hardware when
> the driver starts and use that if present. But, that might mean that
> you'd get different modes depending on whether the machine booted with
> the lid closed or open, which seems like a bad plan.

More than that, I think eDP *requires* an EDID, and it sounds like even
the Air has one (and if any machine didn't, you know it would be an
Apple).

So I'm definitely in favor of this change.

-- 
Jesse Barnes, Intel Open Source Technology Center