Re: [Intel-gfx] [PATCH 04/21] drm/i915: Only use VBT panel mode oneDP if no EDID is found
From: Jesse Barnes
Date: Mon Oct 03 2011 - 16:43:14 EST
On Fri, 30 Sep 2011 10:58:26 -0700
Keith Packard <keithp@xxxxxxxxxx> wrote:
> On Fri, 30 Sep 2011 18:32:35 +0200, Daniel Vetter <daniel@xxxxxxxx> 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
So I'm definitely in favor of this change.
Jesse Barnes, Intel Open Source Technology Center
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