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).
Numerous people have tested this and reported it working (as seen in the bug report thread) which isn't surprising considering it's a basic modification of Chris Wilsons work.
Any comments, or anybody willing to include this patch?
Thanks, Ben Brewer (CodeThink)
On Fri, 29 Jul 2011 13:55:35 +0100, Ben Brewer ben.brewer@codethink.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.
On Fri, 29 Jul 2011 11:45:28 -0700, Keith Packard keithp@keithp.com wrote: Non-text part: multipart/signed
On Fri, 29 Jul 2011 13:55:35 +0100, Ben Brewer ben.brewer@codethink.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.
It's not meant to be and it causes havoc, from wavy/blurry output to no sync. The other part of the patch on that bug was to walk the crtcs and turn off SSC on the shared refclk if any output could not handle SSC. At that point, an objection was raised that we shouldn't even be touching the refclk if any output was currently being driven from it. -Chris
On Fri, 29 Jul 2011 20:02:27 +0100, Chris Wilson chris@chris-wilson.co.uk wrote:
It's not meant to be and it causes havoc, from wavy/blurry output to no sync. The other part of the patch on that bug was to walk the crtcs and turn off SSC on the shared refclk if any output could not handle SSC. At that point, an objection was raised that we shouldn't even be touching the refclk if any output was currently being driven from it.
So the correct fix is to turn off everything, disable SSC and turn everything back on? That seems tedious, but not impossible (just DPMS off the world, then DPMS everything back on...).
On Friday, July 29, 2011, Keith Packard wrote:
On Fri, 29 Jul 2011 13:55:35 +0100, Ben Brewer
ben.brewer@codethink.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.
A pure digital output, where its a high speed serial transfer of bytes to the monitor, is generally not going to be sensitive the any SS used unless the baud rate slams around too much on a byte to byte basis. Since SS is generally just a few percent, even that should not be a problem. But feeding analog signals up a video cable to a vga monitor, and the pixel time is wobbling by 2 to 5% and you won't have much of a viewable image left. The analog signals, vga etc, needs their own video clock, a dead stable one.
We used to spend $10k-30k$ just so we could take the signal from a mechanically unstable vcr that only cost us $10k, and make it stable enough to work on your home tv's without the top of the pix waving around like hurricane Katrina was in the neighborhood.
Hardware design error in the Dell?
Cheers, gene
On Fri, 29 Jul 2011 18:01:39 -0400, Gene Heskett gene.heskett@gmail.com wrote:
On Friday, July 29, 2011, Keith Packard wrote:
On Fri, 29 Jul 2011 13:55:35 +0100, Ben Brewer
ben.brewer@codethink.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 :-)
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