Re: [Intel-gfx] [PATCH 1/1] Let userspace know if they can trust timeslicing by including it as part of the I915_PARAM_HAS_SCHEDULER::I915_SCHEDULER_CAP_TIMESLICING

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 27/05/2021 11:13, Daniel Vetter wrote:
> On Wed, May 26, 2021 at 11:20:13AM +0100, Tvrtko Ursulin wrote:
>>
>> On 25/05/2021 15:47, Daniel Vetter wrote:
>>> On Tue, May 25, 2021 at 03:19:47PM +0100, Tvrtko Ursulin wrote:
>>>>
>>>> + dri-devel as per process
>>>>
>>>> On 25/05/2021 14:55, Tejas Upadhyay wrote:
>>>>> v2: Only declare timeslicing if we can safely preempt userspace.
>>>>
>>>> Commit message got butchered up somehow so you'll need to fix that at some
>>>> point.
>>>>
>>>> Regards,
>>>>
>>>> Tvrtko
>>>>
>>>>> Fixes: 8ee36e048c98 ("drm/i915/execlists: Minimalistic timeslicing")
>>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>> ---
>>>>>     drivers/gpu/drm/i915/gt/intel_engine_user.c | 1 +
>>>>>     include/uapi/drm/i915_drm.h                 | 1 +
>>>>>     2 files changed, 2 insertions(+)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/i915/gt/intel_engine_user.c b/drivers/gpu/drm/i915/gt/intel_engine_user.c
>>>>> index 3cca7ea2d6ea..12d165566ed2 100644
>>>>> --- a/drivers/gpu/drm/i915/gt/intel_engine_user.c
>>>>> +++ b/drivers/gpu/drm/i915/gt/intel_engine_user.c
>>>>> @@ -98,6 +98,7 @@ static void set_scheduler_caps(struct drm_i915_private *i915)
>>>>>     		MAP(HAS_PREEMPTION, PREEMPTION),
>>>>>     		MAP(HAS_SEMAPHORES, SEMAPHORES),
>>>>>     		MAP(SUPPORTS_STATS, ENGINE_BUSY_STATS),
>>>>> +		MAP(TIMESLICE_BIT, TIMESLICING),
>>>>>     #undef MAP
>>>>>     	};
>>>>>     	struct intel_engine_cs *engine;
>>>>> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
>>>>> index c2c7759b7d2e..af2212d6113c 100644
>>>>> --- a/include/uapi/drm/i915_drm.h
>>>>> +++ b/include/uapi/drm/i915_drm.h
>>>>> @@ -572,6 +572,7 @@ typedef struct drm_i915_irq_wait {
>>>>>     #define   I915_SCHEDULER_CAP_PREEMPTION	(1ul << 2)
>>>>>     #define   I915_SCHEDULER_CAP_SEMAPHORES	(1ul << 3)
>>>>>     #define   I915_SCHEDULER_CAP_ENGINE_BUSY_STATS	(1ul << 4)
>>>>> +#define   I915_SCHEDULER_CAP_TIMESLICING	(1ul << 5)
>>>
>>> Since this is uapi I think we should at least have some nice kerneldoc
>>> that explains what exactly this is, what for (link to userspace) and all
>>> that. Ideally also minimally filing in the gaps in our uapi docs for stuff
>>> this references.
>>
>> IIUC there is no userspace apart from IGT needing it not to fail scheduling
>> tests on ADL.
>>
>> Current tests use "has preemption + has semaphores" as a proxy to answer the
>> "does the kernel support timeslicing" question. This stops working with the
>> Guc backend because GuC decided not to support semaphores (for reasons yet
>> unknown, see other thread), so explicit "has timeslicing" flag is needed in
>> order for tests to know that GuC is supposed to support timeslicing, even if
>> it doesn't use semaphores for inter-ring synchronisation.
> 
> Since this if for igt only: Cant we do just extend the check in igt with
> an || GEN >= 12? I really hope that our future hw will continue to support
> timeslicing ...

Not the gen 12 check, but possible I think. Explicit feature test would be better, but if definitely not allowed then along the lines of:

has_timeslicing =
	(has_preemption && has_semaphores) || uses_guc_submission;

Regards,

Tvrtko
  
> Also if it's not there yet, a shared helper to check for that (like we're
> adding for relocations and stuff like that right now).
> -Daniel
>