linux-kernel - Re: i915 SSC Patch

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Date:	Fri, 29 Jul 2011 15:18:05 -0700
From:	Keith Packard <keithp@...thp.com>
To:	Gene Heskett <gene.heskett@...il.com>,
	Ben Brewer <ben.brewer@...ethink.co.uk>,
	dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: Re: i915 SSC Patch

On Fri, 29 Jul 2011 18:01:39 -0400, Gene Heskett <gene.heskett@...il.com> wrote:
> On Friday, July 29, 2011, Keith Packard wrote:
> >On Fri, 29 Jul 2011 13:55:35 +0100, Ben Brewer 
> <ben.brewer@...ethink.co.uk> wrote:
> >> I've added a global SSC (Spread Spectrum Clock) parameter to the i915
> >> driver, since having SSC enabled breaks (distorts) VGA output on some
> >> Core i5/i7 chips (see
> >> https://bugs.freedesktop.org/show_bug.cgi?id=38750). SSC is still
> >> enabled by default so the behaviour won't change but setting the
> >> global_use_ssc parameter will turn this feature off and allow affected
> >> devices to function correctly (notably the Dell Vostro 3300).
> >
> >The question I have is why is SSC enabled on the VGA output at all? I
> >don't see any way VGA could ever tolerate it.
> 
> Something does not make sense here Keith, so I'm with you, and my 
> background is from about 60 years in tv maintenance and 45 in
> broadcasting.

Right, I think the basic problem is that we aren't switching SSC on and
off based on whether there's an output which can't tolerate it. Making
this user-configurable doesn't make any sense, it clearly needs to be
done in the driver automatically, based on whether there's an analog
output running (VGA or TV).

> Hardware design error in the Dell?

Nope, just a driver bug :-)

-- 
keith.packard@...el.com

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