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<b><a class="bz_bug_link
bz_status_ASSIGNED "
title="ASSIGNED --- - [snb dp hotplug] Pipe B, PCH transcoder B FIFO underrun"
href="https://bugs.freedesktop.org/show_bug.cgi?id=70254#c29">Comment # 29</a>
on <a class="bz_bug_link
bz_status_ASSIGNED "
title="ASSIGNED --- - [snb dp hotplug] Pipe B, PCH transcoder B FIFO underrun"
href="https://bugs.freedesktop.org/show_bug.cgi?id=70254">bug 70254</a>
from <span class="vcard"><a class="email" href="mailto:ville.syrjala@linux.intel.com" title="Ville Syrjala <ville.syrjala@linux.intel.com>"> <span class="fn">Ville Syrjala</span></a>
</span></b>
<pre>Created <span class=""><a href="attachment.cgi?id=92540" name="attach_92540" title="Patch to allow changing watermark latency values">attachment 92540</a> <a href="attachment.cgi?id=92540&action=edit" title="Patch to allow changing watermark latency values">[details]</a></span> <a href='page.cgi?id=splinter.html&bug=70254&attachment=92540'>[review]</a>
Patch to allow changing watermark latency values
This patch allows changing the latency values we use for computing the
watermarks.
It adds three new debugfs files. "i915_pri_wm_latency" being the one we're
interested in here.
Reading the file should give similar output as the kernel log had. So in this
case it should look like this:
# cat i915_pri_wm_latency
Primary WM0 latency 7 (0.7 usec)
Primary WM1 latency 3 (1.5 usec)
Primary WM2 latency 4 (2.0 usec)
Primary WM3 latency 22 (11.0 usec)
What you could then do is write new latency values to the file. Let's say we
try to double the latency values:
# echo '14 6 8 44' > i915_pri_wm_latency
Now reading the file again should show the new values. To actually make the
system use them you'd need to force a modeset on all the displays.
"xset dpms force off; xset dpms force on" should be enough for that. After this
is done you should see some change in the 0x45100 and 0x45104 registers.
And then it should just be a matter of trying to cause another underrun, and
increasing the latency values until they no longer occur.</pre>
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