[Intel-gfx] Possible 4.5 i915 Skylake regression

[Andy Lutomirski]

On Wed, Feb 17, 2016 at 8:18 AM, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Tue, Feb 16, 2016 at 09:26:35AM -0800, Andy Lutomirski wrote:
>> On Tue, Feb 16, 2016 at 9:12 AM, Andy Lutomirski <luto at amacapital.net> wrote:
>> > On Tue, Feb 16, 2016 at 8:12 AM, Daniel Vetter <daniel at ffwll.ch> wrote:
>> >> On Mon, Feb 15, 2016 at 06:58:33AM -0800, Andy Lutomirski wrote:
>> >>> On Sun, Feb 14, 2016 at 6:59 PM, Andy Lutomirski <luto at kernel.org> wrote:
>> >>> > Hi-
>> >>> >
>> >>> > On 4.5-rc3 on a Dell XPS 13 9350 (Skylake i915, no nvidia on this
>> >>> > model), shortly after resume, I saw a single black flash on the
>> >>> > screen.  The log said:
>> >>> >
>> >>> > [Feb13 07:05] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR*
>> >>> > CPU pipe A FIFO underrun
>> >>> >
>> >>> > I haven't seen this on 4.4.
>> >>> >
>> >>> > I'd be happy to dig up debugging info, but I don't know what would be
>> >>> > useful.  I have no i915 module options set.
>> >>>
>> >>> It's flashing quite frequently now, although I seem to get the
>> >>> underrun warning only once per resume.
>> >>
>> >> We shut up the warning irq source to avoid hijacking an entire cpu core
>> >> ;-)
>> >>
>> >> There's a fix from Matt right after 4.5-rc4 in Linus' branch. I'm hoping
>> >> that should help.
>> >
>> > Do you mean:
>> >
>> > commit e2e407dc093f530b771ee8bf8fe1be41e3cea8b3
>> > Author: Matt Roper <matthew.d.roper at intel.com>
>> > Date:   Mon Feb 8 11:05:28 2016 -0800
>> >
>> >     drm/i915: Pretend cursor is always on for ILK-style WM calculations (v2)
>> >
>> > If so, it didn't help.  I'm currently doing a full rebuild just in
>> > case I messed something up, though.
>> >
>>
>> Definitely not fixed.  It seems to be okay after a reboot until the
>> first suspend/resume.
>>
>> This happened after resuming.  Five cents says it's the root cause.
>
> That's interesting, but doesn't ring a bell unfortunately. Can you try to
> attempt a bisect?

I probably can, but it's very slow.  Is there a reasonably
straightforward way to instrument the watermark computation to see
what's going wrong?  I'm reasonably confident that the bug is in the
resume code or in something that only happens on resume, since I still
haven't seen underruns after rebooting before suspending.

--Andy