Comment 64 for bug 1239186

Created attachment 92540
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.