Re: [PATCH 1/2] drm/i915: Increase render/media power gating hysteresis for gen9+

[Sagar Arun Kamble <sagar.a.kamble@intel.com>]

On 1/21/2018 5:55 PM, Chris Wilson wrote:
> Quoting Sagar Arun Kamble (2018-01-21 11:01:23)
>>
>> On 1/20/2018 3:01 PM, Chris Wilson wrote:
>>> On gen9+, after an idle period the HW will disable the entire power well
>>> to conserve power (by preventing current leakage). It takes around a 100
>>> microseconds to bring the power well back online afterwards. With the
>>> current hysteresis value of 25us, we do not have sufficient time to
>>> respond to an interrupt and schedule the next execution before the HW
>>> powers itself down. (At present, we prevent this by grabbing the
>>> forcewake for prolonged periods of time, but that is overkill fixed in
>>> the next patch.) The minimum we want to set the power gating hysteresis
>>> to is the length of time it takes us to service the GPU, which across a
>>> broad spectrum of machines is about 250us.
>>>
>>> (Note this also brings guc latency into the same ballpark as execlists.)
>>>
>>> Testcase: igt/gem_exec_nop/sequential
>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>>> Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
>>> Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
>>> Cc: Michel Thierry <michel.thierry@intel.com>
>>> Cc: Michal Winiarski <michal.winiarski@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
>>> ---
>>>    drivers/gpu/drm/i915/intel_pm.c | 4 ++--
>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
>>> index 1db79a860b96..6748d3efb537 100644
>>> --- a/drivers/gpu/drm/i915/intel_pm.c
>>> +++ b/drivers/gpu/drm/i915/intel_pm.c
>>> @@ -6627,8 +6627,8 @@ static void gen9_enable_rc6(struct drm_i915_private *dev_priv)
>>>        I915_WRITE(GEN6_RC_SLEEP, 0);
>>>    
>>>        /* 2c: Program Coarse Power Gating Policies. */
>>> -     I915_WRITE(GEN9_MEDIA_PG_IDLE_HYSTERESIS, 25);
>>> -     I915_WRITE(GEN9_RENDER_PG_IDLE_HYSTERESIS, 25);
>>> +     I915_WRITE(GEN9_MEDIA_PG_IDLE_HYSTERESIS, 250);
>>> +     I915_WRITE(GEN9_RENDER_PG_IDLE_HYSTERESIS, 250);
>> As per bspec this is in 1.28usec units so it should be 195.
> Both numbers pulled out of the air, just read us above as approx.us.
ok ... Interestingly spec too says 25us and also say that it is actually 
32us :)
> -Chris

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