[Intel-gfx] [PATCH v5] drm/i915/icl: Preempt-to-idle support in execlists.

[Lis, Tomasz]

On 2018-10-16 12:53, Joonas Lahtinen wrote:
> Quoting Tomasz Lis (2018-10-15 20:29:18)
>> The patch adds support of preempt-to-idle requesting by setting a proper
>> bit within Execlist Control Register, and receiving preemption result from
>> Context Status Buffer.
>>
>> Preemption in previous gens required a special batch buffer to be executed,
>> so the Command Streamer never preempted to idle directly. In Icelake it is
>> possible, as there is a hardware mechanism to inform the kernel about
>> status of the preemption request.
>>
>> This patch does not cover using the new preemption mechanism when GuC is
>> active.
>>
>> v2: Added needs_preempt_context() change so that it is not created when
>>      preempt-to-idle is supported. (Chris)
>>      Updated setting HWACK flag so that it is cleared after
>>      preempt-to-dle. (Chris, Daniele)
>>      Updated to use I915_ENGINE_HAS_PREEMPTION flag. (Chris)
>>
>> v3: Fixed needs_preempt_context() change. (Chris)
>>      Merged preemption trigger functions to one. (Chris)
>>      Fixed conyext state tonot assume COMPLETED_MASK after preemption,
>>      since idle-to-idle case will not have it set.
>>
>> v4: Simplified needs_preempt_context() change. (Daniele)
>>      Removed clearing HWACK flag in idle-to-idle preempt. (Daniele)
>>
>> v5: Renamed inject_preempt_context(). (Daniele)
>>      Removed duplicated GEM_BUG_ON() on HWACK (Daniele)
>>
>> Bspec: 18922
>> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
>> Cc: Chris Wilson <chris at chris-wilson.co.uk>
>> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
>> Cc: Michal Winiarski <michal.winiarski at intel.com>
>> Cc: Mika Kuoppala <mika.kuoppala at intel.com>
>> Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> This R-b was on v4, and should be indicated with # v4 comment.
>
> The commit message doesn't say much about why preempting to idle is
> beneficial? The pre-Gen11 codepath needs to be maintained anyway.
>
> Regards, Joonas
The benefit is one less context switch - there is no "preempt context".
-Tomasz