Re: [PATCH] drm/i915: Workaround to avoid lite restore with HEAD==TAIL

[Michel Thierry <michel.thierry@intel.com>]

On 4/10/2015 11:01 AM, Daniel Vetter wrote:
> On Fri, Apr 10, 2015 at 10:32:33AM +0100, Michel Thierry wrote:
>> WaIdleLiteRestore is an execlists-only workaround, and requires the driver
>> to ensure that any context always has HEAD!=TAIL when attempting lite
>> restore.
>>
>> Add two extra MI_NOOP instructions at the end of each request, but keep
>> the requests tail pointing before the MI_NOOPs.
>>
>> If we submit a context to the ELSP which has previously been submitted,
>> move the tail pointer past the MI_NOOPs. This ensures HEAD!=TAIL.
>>
>> Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
>> Signed-off-by: Michel Thierry <michel.thierry@intel.com>
> Is there an igt to provoke this? I'm thinking of submitting tiny noop
> batches on 2 alternating contexts. That should be able to make this go
> boom pretty reliable, we only need a slight delay in processing the
> execlist completion interrupt.
> -Daniel
I've seen it when running multiple contexts in parallel or when the 
wa-batchbuffer was enabled.
I'll write an igt for it, we only need several small batches queued to 
trigger the light restore.

--Michel

>> ---
>>   drivers/gpu/drm/i915/i915_gem.c  | 11 +++++++++++
>>   drivers/gpu/drm/i915/intel_lrc.c | 29 +++++++++++++++++++++++++++++
>>   2 files changed, 40 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>> index ca522c9..a44070a 100644
>> --- a/drivers/gpu/drm/i915/i915_gem.c
>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>> @@ -2382,6 +2382,17 @@ int __i915_add_request(struct intel_engine_cs *ring,
>>   	request->head = request_start;
>>   	request->tail = intel_ring_get_tail(ringbuf);
>>   
>> +	if (i915.enable_execlists &&
>> +	    (IS_GEN8(ring->dev) || IS_GEN9(ring->dev))) {
>> +		/*
>> +		 * Here we add two extra NOOPs as padding to avoid
>> +		 * lite restore of a context with HEAD==TAIL.
>> +		 */
>> +		intel_logical_ring_emit(ringbuf, MI_NOOP);
>> +		intel_logical_ring_emit(ringbuf, MI_NOOP);
>> +		intel_logical_ring_advance(ringbuf);
>> +	}
>> +
>>   	/* Whilst this request exists, batch_obj will be on the
>>   	 * active_list, and so will hold the active reference. Only when this
>>   	 * request is retired will the the batch_obj be moved onto the
>> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
>> index f18f96c..8acedca 100644
>> --- a/drivers/gpu/drm/i915/intel_lrc.c
>> +++ b/drivers/gpu/drm/i915/intel_lrc.c
>> @@ -413,6 +413,26 @@ static void execlists_context_unqueue(struct intel_engine_cs *ring)
>>   		}
>>   	}
>>   
>> +	if (IS_GEN8(ring->dev) || IS_GEN9(ring->dev)) {
>> +		/*
>> +		 * WaIdleLiteRestore: make sure we never cause a lite
>> +		 * restore with HEAD==TAIL
>> +		 */
>> +		if (req0 && req0->elsp_submitted == 1) {
>> +			/*
>> +			 * Consume the buffer NOOPs to ensure HEAD != TAIL when
>> +			 * submitting. elsp_submitted can only be >1 after
>> +			 * reset, in which case we don't need the workaround as
>> +			 * a lite restore will not occur.
>> +			 */
>> +			struct intel_ringbuffer *ringbuf;
>> +
>> +			ringbuf = req0->ctx->engine[ring->id].ringbuf;
>> +			req0->tail += 8;
>> +			req0->tail &= ringbuf->size - 1;
>> +		}
>> +	}
>> +
>>   	WARN_ON(req1 && req1->elsp_submitted);
>>   
>>   	execlists_submit_contexts(ring, req0->ctx, req0->tail,
>> @@ -829,6 +849,15 @@ static int intel_logical_ring_begin(struct intel_ringbuffer *ringbuf,
>>   	struct drm_i915_private *dev_priv = dev->dev_private;
>>   	int ret;
>>   
>> +	if (IS_GEN8(dev) || IS_GEN9(dev)) {
>> +		/*
>> +		 * Reserve space for 2 NOOPs at the end of each request to be
>> +		 * used as a workaround for not being allowed to do lite
>> +		 * restore with HEAD==TAIL.
>> +		 */
>> +		num_dwords += 2;
>> +	}
>> +
>>   	ret = i915_gem_check_wedge(&dev_priv->gpu_error,
>>   				   dev_priv->mm.interruptible);
>>   	if (ret)
>> -- 
>> 2.1.1
>>
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