[REGRESSION, i915]: Periodic stalls with 2.6.36-rc2

[Chris Wilson]

On Wed, 25 Aug 2010 21:15:06 +0200, Peter Stuge <peter at stuge.se> wrote:
> In general, Xv performance with KMS leaves me with a feeling that
> something is not quite right in terms of playback, both with mplayer
> and vlc. But while movies are nice, this stalling issue is more
> important.

That should now be fixed in -intel, we allocating a fresh buffer for every
frame and incurring massive clflush overhead. However, we have we replaced
it with a worse bug...
 
> drm-intel.git as of 07-21 has sortof worked for me, but could not
> suspend. Later drm-intel.git would come up with the panel disabled,
> so basically make the system unusable. I've been using 07-21 for
> about a month, and just avoiding suspending.
> 
> I'm now running drm-intel.git/drm-intel-next (which seems to not get
> the activity first?) and the latest commit is Linus 2.6.36-rc2. I've
> merged wireless-testing.git on top of this for ath9k woes.
> 
> I get the stall precisely every 10 seconds. In VT mode the system
> stalls for about a second (the cursor stops blinking) and if I'm
> typing during the stall then my keypresses appear correctly after the
> stall ends. No errors in dmesg. Should I set drm.debug to something?

This is caused by the hotplug polling code taking around 600ms to
determine that the VGA is not connected.

https://bugs.freedesktop.org/show_bug.cgi?id=29536

> If I'm running X (I've tried xf86-video-intel 2.12.0 and current .git
> master) then my keypresses appear twice after the stall ends, but
> honouring shift. If I hold shift, type X, release shift during a
> stall then I'll get "Xx" after the stall ends. This makes X
> practically unusable for me, so I'm at a plain VT for now.
> 
> Suspend works both in VT and in X, so some things have definately
> improved for me since 07-21. :)
> 
> I've just upgraded my userspace and have xorg-server-1.8.2. No Xv
> image with either xf86-video-intel driver. I haven't tried this
> server with the 07-21 kernel to find out if Xv works there. The
> stalling is more important.

https://bugs.freedesktop.org/show_bug.cgi?id=29574 (I think, Daniel
reported something closer to your issue on IRC, but I don't have an actual
bug for that.)

> Chris' patch to i915_irq.c disabling wake-ups has no effect on the
> stalls in VT mode for me.
> 
> I haven't bisected, would that be a big help? I'll probably have to
> spend a fair amount of time on that, since the latest previous kernel
> I could get to start is over a month old. I've tried drm-intel.git a
> couple of times over this month, but for long periods of time there
> were no changes. (Should I look elsewhere?)

No, sounds like you've hit almost all of the recent 855 regressions. But
you have a cursor, that is more than most people!
 
> I also get intermittent FIFO errors, but I don't know how relevant
> they are for this issue. It seems like the stalling is a very
> distinct issue.

Actually reporting the FIFO issue is new, the problem has been around for
a while though. The underlying issue is that we have a very pessimistic
guess for the memory fetch and so estimate that we require more FIFO
entries than is available on the hardware. The challenge is to build a
table of suitable memory latencies that we can use for a more accurate
guess, and save power.

-- 
Chris Wilson, Intel Open Source Technology Centre