Re: [PATCH for 3.12/-fixes 2/2] drm/i915/dp: workaround BIOS eDP bpp clamping issue

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Oct 21, 2013 at 10:52:07AM +0300, Jani Nikula wrote:
> This isn't a real fix to the problem, but rather a stopgap measure while
> trying to find a proper solution.
> 
> There are several laptops out there that fail to light up the eDP panel
> in UEFI boot mode. They seem to be mostly IVB machines, including but
> apparently not limited to Dell XPS 13, Asus TX300, Asus UX31A, Asus
> UX32VD, Acer Aspire S7. They seem to work in CSM or legacy boot.
> 
> The difference between UEFI and CSM is that the BIOS provides a
> different VBT to the kernel. The UEFI VBT typically specifies 18 bpp and
> 1.62 GHz link for eDP, while CSM VBT has 24 bpp and 2.7 GHz link. We end
> up clamping to 18 bpp in UEFI mode, which we can fit in the 1.62 Ghz
> link, and for reasons yet unknown fail to light up the panel.
> 
> Dithering from 24 to 18 bpp itself seems to work; if we use 18 bpp with
> 2.7 GHz link, the eDP panel lights up. So essentially this is a link
> speed issue, and *not* a bpp clamping issue.
> 
> The bug raised its head since
> commit 657445fe8660100ad174600ebfa61536392b7624
> Author: Daniel Vetter <daniel.vetter@ffwll.ch>
> Date:   Sat May 4 10:09:18 2013 +0200
> 
>     Revert "drm/i915: revert eDP bpp clamping code changes"
> 
> which started clamping bpp *before* computing the link requirements, and
> thus affecting the required bandwidth. Clamping after the computations
> kept the link at 2.7 GHz.
> 
> Even though the BIOS tells us to use 18 bpp through the VBT, it happily
> boots up at 24 bpp and 2.7 GHz itself! Use this information to
> selectively ignore the VBT provided value.
> 
> We can't ignore the VBT eDP bpp altogether, as there are other laptops
> that do require the clamping to be used due to EDID reporting higher bpp
> than the panel can support.
> 
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=59841
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67950
> Tested-by: Ulf Winkelvos <ulf@winkelvos.de>
> Tested-by: jkp <jkp@iki.fi>
> CC: stable@vger.kernel.org
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Both merged to -fixes, thanks for the quick backport. I've added the note
about we need the first patch to the commit message and also added the cc:
stable tag.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch