[Intel-gfx] [RFC 0/3] drm/i915: expose fifo pipe underrun counts

[Joe Konno]

On 01/26/2016 12:51 PM, Daniel Vetter wrote:
> On Tue, Jan 26, 2016 at 11:36:54AM -0800, Joe Konno wrote:
>> From: Joe Konno <joe.konno at intel.com>
>>
>> In tracking down a watermark bug, I discovered the pch and cpu underrun
>> interrupt handlers would disable themselves after initial reports to prevent an
>> interrupt/dmesg storm. Storms are bad, but underrun interrupt handling should
>> not cease. For my case, I need to be able to count pch and cpu underruns for
>> each pipe or transcoder. Displaying this information in the 'i915_display_info'
>> node seemed the best course of action.
>>
>> In order to do this, however, I had to revisit some long-standing behaviors in
>> the underrun interrupt handlers. One problem became three. Thanks in advance
>> for your review and feedback.
>>
>> Requesting comment on the following solutions I came up with (corresponding to
>> each patch in the series):
>>
>>   1. provide simple 'getter' mechanisms for pch and cpu underrun reporting
>>      ("is it enabled?")-- and base dmesg output on the answer to that question;
>>
>>   2. don't allow the interrupt handlers to disable or filter themselves (and
>>      prevent accurate counting); and finally
>>
>>   3. atomically-incremented pch and cpu underrun counters, with those counters
>>      displayed in debugfs i915_display_info per-pipe, per-transcoder
>>
>> For: https://bugs.freedesktop.org/show_bug.cgi?id=93865
> 
> It's more complicated than this, replied with the technicalities to patch
> 2. But what I've forgotten to ask: What do you want to use this for? We
> make sure that after a full modeset underrun reporting state is restored,
> so you can retest for a given bug essentially forever, with no need to
> reboot.

I see a correlation between pipe underruns and display flicker-- that's
the particular case I'm working presently, so an underrun counter (in
whatever form) is extremely useful. Assuming, of course, I'm barking up
the right tree.

Such a counter seems a reliable metric for expressing "how bad"
insert-display-issue-here is. Or, "how much better/worse" said issue is
after hacking or applying patch(es).

> And we've also become a lot better at fixing the existing underruns, so
> nowadays fifo reporting won't be disable right away even before you
> managed to display the very first frame.
> -Daniel