linux-kernel - Re: [Intel-gfx] [PATCH 1/2] drm/i915: Force explicit bpp selection for intel_dp_link

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Message-ID: <20120125215245.GJ3896@phenom.ffwll.local>
Date:	Wed, 25 Jan 2012 22:52:45 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Keith Packard <keithp@...thp.com>
Cc:	intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	Lubos Kolouch <lubos.kolouch@...il.com>
Subject: Re: [Intel-gfx] [PATCH 1/2] drm/i915: Force explicit bpp selection
 for intel_dp_link_required

On Wed, Jan 25, 2012 at 08:16:25AM -0800, Keith Packard wrote:
> It is never correct to use intel_crtc->bpp in intel_dp_link_required,
> so instead pass an explicit bpp in to this function. This patch
> only supports 18bpp and 24bpp modes, which means that 10bpc modes will
> be computed incorrectly. Fixing that will require more extensive
> changes, and so must be addressed separately from this bugfix.
> 
> intel_dp_link_required is called from intel_dp_mode_valid and
> intel_dp_mode_fixup.
> 
> * intel_dp_mode_valid is called to list supported modes; in this case,
>   the current crtc values cannot be relevant as the modes in question
>   may never be selected. Thus, using intel_crtc->bpp is never right.
> 
> * intel_dp_mode_fixup is called during mode setting, but it is run
>   well before ironlake_crtc_mode_set is called to set intel_crtc->bpp,
>   so using intel_crtc-bpp in this path can only ever get a stale
>   value.
> 
> Cc: Lubos Kolouch <lubos.kolouch@...il.com>
> Cc: Adam Jackson <ajax@...hat.com>
> Signed-off-by: Keith Packard <keithp@...thp.com>

Afaics this is correct and should fix quite a few "dp doesn't light up
issue" (in combination with 6bpc auto-dither code that's already there). I
think all the open issues are only about how to make this less
pessimistic and more generic, i.e. patch 2.

Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
-- 
Daniel Vetter
Mail: daniel@...ll.ch
Mobile: +41 (0)79 365 57 48
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