Re: [PATCH 02/12] drm/i915/gen9: Add WaOCLCoherentLineFlush

[Arun Siluvery <arun.siluvery@linux.intel.com>]

On 28/09/2015 17:02, Ville Syrjälä wrote:
> On Mon, Sep 28, 2015 at 04:51:52PM +0100, Arun Siluvery wrote:
>> On 25/09/2015 18:09, Ville Syrjälä wrote:
>>> On Fri, Sep 25, 2015 at 02:33:36PM +0100, Arun Siluvery wrote:
>>>> Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
>>>> ---
>>>>    drivers/gpu/drm/i915/intel_pm.c | 4 ++++
>>>>    1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
>>>> index ab5ac5e..093a5e4 100644
>>>> --- a/drivers/gpu/drm/i915/intel_pm.c
>>>> +++ b/drivers/gpu/drm/i915/intel_pm.c
>>>> @@ -63,6 +63,10 @@ static void gen9_init_clock_gating(struct drm_device *dev)
>>>>    	/* WaDisableKillLogic:bxt,skl */
>>>>    	I915_WRITE(GAM_ECOCHK, I915_READ(GAM_ECOCHK) |
>>>>    		   ECOCHK_DIS_TLB);
>>>> +
>>>> +	/* WaOCLCoherentLineFlush:skl,bxt */
>>>> +	I915_WRITE(GEN8_L3SQCREG4, (I915_READ(GEN8_L3SQCREG4) |
>>>> +				    GEN8_LQSC_FLUSH_COHERENT_LINES));
>>>
>>> According to Bspec + w/a db this should be done for BDW too (actually
>>> BSpec shows it for BDW only?). If that's the case, then we should be
>>> able to kill gen8_emit_flush_coherentl3_wa(), no? Well, as long as
>>> someone goes and adds the DC flush to the normal post batch flush.
>>>
>> Yes this is applicable for BDW also but I wanted to keep only Gen9
>> patches in this series. I will send separate patch for BDW.
>> We would still need gen8_emit_flush_coherentl3_wa() because WA requires
>> that the flush need to happen from the WA batch itself during context
>> switch.
>
> If we already flush all coherent lines out from DC after each batch,
> how can there still be coherent lines in the DC on a context switch?
>
> But spelling that out made me think that mid-batch preemption would
> still need it I suppose. But w/o preemption I see no reason why it
> would be needed.
>
Yes, this is to account preemption case (sorry I should've mentioned 
earlier). Most of the workarounds in WA batch are to account for 
preemption related issues.

regards
Arun


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