Re: [PATCH v3] drm/i915: Workaround to avoid lite restore with HEAD==TAIL

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

On 4/15/2015 5:40 PM, Chris Wilson wrote:
> On Wed, Apr 15, 2015 at 05:17:13PM +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. We may not need to
>> executed them, and this is why request->tail must be sampled before adding
>> these extra instructions.
>>
>> 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.
>>
>> v2: Move overallocation to gen8_emit_request, and added note about
>> sampling request->tail in commit message (Chris).
>>
>> v3: Remove redundant request->tail assignment in __i915_add_request, in
>> lrc mode this is already set in execlists_context_queue.
>> Do not add wa implementation details inside gem (Chris).
>>
>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>> Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
>> Signed-off-by: Michel Thierry <michel.thierry@intel.com>
>> ---
>>   drivers/gpu/drm/i915/i915_gem.c  |  3 ++-
>>   drivers/gpu/drm/i915/intel_lrc.c | 35 ++++++++++++++++++++++++++++++++++-
>>   2 files changed, 36 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>> index 3d5a5a8..980e17c 100644
>> --- a/drivers/gpu/drm/i915/i915_gem.c
>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>> @@ -2400,10 +2400,11 @@ int __i915_add_request(struct intel_engine_cs *ring,
>>   		ret = ring->add_request(ring);
>>   		if (ret)
>>   			return ret;
>> +
>> +		request->tail = intel_ring_get_tail(ringbuf);
>>   	}
>>   
>>   	request->head = request_start;
>> -	request->tail = intel_ring_get_tail(ringbuf);
>>   
>>   	/* Whilst this request exists, batch_obj will be on the
>>   	 * active_list, and so will hold the active reference. Only when this
>> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
>> index f4a5ef9..0296350 100644
>> --- a/drivers/gpu/drm/i915/intel_lrc.c
>> +++ b/drivers/gpu/drm/i915/intel_lrc.c
>> @@ -427,6 +427,26 @@ 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.
> I actually think you can remove the == 1 and hence remove comment since
> the wa is safe to apply in that case as well.
>
> /* Apply the wa NOOPS to prevent ring:HEAD == rq:TAIL as we
>   * resubmit the request. See gen8_emit_request() for where we
>   * prepare the padding after the end of the request.
>   */
Yes, it's safe to apply it after the request has been submitted multiple 
times.
I'll change that and update the comment.

Thanks,

-Michel

>> +			 */
>> +			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,
>> @@ -1289,7 +1309,12 @@ static int gen8_emit_request(struct intel_ringbuffer *ringbuf,
>>   	u32 cmd;
>>   	int ret;
>>   
>> -	ret = intel_logical_ring_begin(ringbuf, request->ctx, 6);
>> +	/*
>> +	 * 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 (WaIdleLiteRestore).
>> +	 */
>> +	ret = intel_logical_ring_begin(ringbuf, request->ctx, 8);
>>   	if (ret)
>>   		return ret;
>>   
>> @@ -1307,6 +1332,14 @@ static int gen8_emit_request(struct intel_ringbuffer *ringbuf,
>>   	intel_logical_ring_emit(ringbuf, MI_NOOP);
>>   	intel_logical_ring_advance_and_submit(ringbuf, request->ctx, request);
>>   
>> +	/*
>> +	 * 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);
>> +
> Ok, looks better.
> -Chris
>
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