[Intel-gfx] [PATCH] drm/i915: always set FDI composite sync bit

[Jesse Barnes]

On Tue, 11 Oct 2011 10:10:24 -0400
Adam Jackson <ajax at redhat.com> wrote:

> On 10/10/11 7:22 PM, Keith Packard wrote:
> > On Mon, 10 Oct 2011 14:28:52 -0700, Jesse Barnes<jbarnes at virtuousgeek.org>  wrote:
> >
> >> It's needed for 3 pipe support as well as just regular functionality
> >> (e.g. DisplayPort).
> >
> > Any explanation on how you get sync without this? As in, why did this
> > ever work?
> 
> To a first approximation, it didn't.  I was never able to get anything 
> reliably lit up besides the boot output (and only that one; hotplug 
> after was a disaster).
> 
> I don't have register dumps handy to compare BIOS setup with this 
> change, but I could if needed.
> 
> It still seems entirely magical and probably wrong in some situations. 
> And I'm thrilled to see that PPT is functionally different from CPT 
> (seriously, stop doing that) instead of just moving bit definitions 
> around (seriously, stop doing that).  But this change is massively 
> better than before.

Without composite sync, FDI needs frame/line sync (the fsync/lsync
mentioned in the docs), which means extra wires between the CPU and
PCH.  Some boards don't have these and so we need to use composite.
For the same reason, if FDI B and C are sharing lanes, we need to use
composite or we won't have enough lines to go around.

I've asked the hardware guys whether using composite on a board that
supports fsync/lsync is ok generally; if so just defaulting to that
everywhere should be fine.

-- 
Jesse Barnes, Intel Open Source Technology Center
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