2016-07-12 19:21 GMT+01:00 Matt Roper matthew.d.roper@intel.com:
On Tue, Jul 12, 2016 at 01:36:03PM -0400, Lyude wrote:
Since the watermark calculations for Skylake are still broken, we're apt to hitting underruns very easily under multi-monitor configurations. While it would be lovely if this was fixed, it's not. Another problem that's been coming from this however, is the mysterious issue of underruns causing full system hangs. An easy way to reproduce this with a skylake system:
- Get a laptop with a skylake GPU, and hook up two external monitors to it
- Move the cursor from the built-in LCD to one of the external displays as quickly as you can
- You'll get a few pipe underruns, and eventually the entire system will just freeze.
After doing a lot of investigation and reading through the bspec, I found the existence of the SAGV, which is responsible for adjusting the system agent voltage and clock frequencies depending on how much power we need. According to the bspec:
"The display engine access to system memory is blocked during the adjustment time. SAGV defaults to enabled. Software must use the GT-driver pcode mailbox to disable SAGV when the display engine is not able to tolerate the blocking time."
The rest of the bspec goes on to explain that software can simply leave the SAGV enabled, and disable it when we use interlaced pipes/have more then one pipe active.
Sure enough, with this patchset the system hangs resulting from pipe underruns on Skylake have completely vanished on my T460s. Additionally, the bspec mentions turning off the SAGV with more then one pipe
enabled
as a workaround for display underruns. While this patch doesn't entirely fix that, it looks like it does improve the situation a little bit so it's likely this is going to be required to make watermarks on Skylake fully functional.
Changes since v2:
- Really apply minor style nitpicks to patch this time
Changes since v1:
- Added comments about this probably being one of the requirements to fixing Skylake's watermark issues
- Minor style nitpicks from Matt Roper
- Disable these functions on Broxton, since it doesn't have an SAGV
Cc: Matt Roper matthew.d.roper@intel.com Cc: Daniel Vetter daniel.vetter@ffwll.ch Cc: Ville Syrjälä ville.syrjala@linux.intel.com Signed-off-by: Lyude cpaul@redhat.com
I don't have a SKL to try this out on (only BXT here), but this matches my interpretation of the current bspec text, so
Reviewed-by: Matt Roper matthew.d.roper@intel.com
I think this also applies to
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94625
Yes, this does sound very similar to the problems I were having in that report. Vacation etc. so haven't found time to try the 4.6 kernel yet, and now I will be travelling until next week. If someone is easily able to prepare a kernel for Fedora 24 with this patch it's more likely I can find time to try it next week (on Dell XPS 9550 with 1 or 2 external monitors).
I should mention that there was a brief period in 4.4.4-4.4.9 or so that seemed to work fairly stable compared to both 4.3 and 4.5.
/Oskar