[Intel-gfx] [PATCH v4 3/3] drm/i915/gt: Timeout when waiting for idle in suspending

[Tvrtko Ursulin]

On 14/11/2023 19:48, Teres Alexis, Alan Previn wrote:
> On Tue, 2023-11-14 at 17:52 +0000, Tvrtko Ursulin wrote:
>> On 14/11/2023 17:37, Teres Alexis, Alan Previn wrote:
>>> On Tue, 2023-11-14 at 17:27 +0000, Tvrtko Ursulin wrote:
>>>> On 13/11/2023 17:57, Teres Alexis, Alan Previn wrote:
>>>>> On Wed, 2023-10-25 at 13:58 +0100, Tvrtko Ursulin wrote:
>>>>>> On 04/10/2023 18:59, Teres Alexis, Alan Previn wrote:
>>>>>>> On Thu, 2023-09-28 at 13:46 +0100, Tvrtko Ursulin wrote:
>>>>>>>> On 27/09/2023 17:36, Teres Alexis, Alan Previn wrote:
>>
> alan:snip
> 
>>> alan: So i did trace back the gt->wakeref before i posted these patches and
>>> see that within these runtime get/put calls, i believe the first 'get' leads
>>> to __intel_wakeref_get_first which calls intel_runtime_pm_get via rpm_get
>>> helper and eventually executes a pm_runtime_get_sync(rpm->kdev); (hanging off
>>> i915_device). (naturally there is a corresponding mirros for the '_put_last').
>>> So this means the first-get and last-put lets the kernel know. Thats why when
>>> i tested this patch, it did actually cause the suspend to abort from kernel side
>>> and the kernel would print a message indicating i915 was the one that didnt
>>> release all refs.
>>
>> Ah that would be much better then.
>>
>> Do you know if everything gets resumed/restored correctly in that case
>> or we would need some additional work to maybe early exit from callers
>> of wait_for_suspend()?
> alan: So assuming we are still discussing about a "potentially new
> future leaked-wakeref bug" (i.e. putting aside the fact that
> Patch #1 + #2 resolves this specific series' bug), based on the
> previous testing we did, after this timeout-bail trigger,
> the suspend flow bails and gt/guc operation does actually continue
> as normal. However, its been a long time since we tested this so
> i am not sure of how accidental-new-future bugs might play to this
> assumption especially if some other subsystem that leaked the rpm
> wakref but that subsystem did NOT get reset like how GuC is reset
> at the end of suspend.
> 
>>
>> What I would also ask is to see if something like injecting a probing
>> failure is feasible, so we can have this new timeout exit path
>> constantly/regularly tested in CI.
> alan: Thats a good idea. In line with this, i would like to point out that
> rev6 of this series has been posted but i removed this Patch #3. However i did
> post this Patch #3 as a standalone patch here: https://patchwork.freedesktop.org/series/126414/
> as i anticipate this patch will truly help with future issue debuggability.
> 
> That said, i shall post a review on that patch with your suggestion to add
> an injected probe error for the suspend-resume flow and follow up on that one
> separately.

Cool! I don't know exactly how to do it but if we come up with way and 
so gain IGT coverage then I am okay with the patch in principle.

Like perhaps some new debugfs api needs to be added to provoke the 
timeout error path on suspend, or something.

Regards,

Tvrtko

> 
>>
>> Regards,
>>
>> Tvrtko
>>
>>> alan: Anyways, i have pulled this patch out of rev6 of this series and created a
>>> separate standalone patch for this patch #3 that we review independently.
>>>
>