Re: [PATCH] drm/i915: Tidy workaround batch buffer emission

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 08/02/2017 13:20, Chris Wilson wrote:
> On Wed, Feb 08, 2017 at 01:13:48PM +0000, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> -static int gen9_init_perctx_bb(struct intel_engine_cs *engine,
>> -			       struct i915_wa_ctx_bb *wa_ctx,
>> -			       uint32_t *batch,
>> -			       uint32_t *offset)
>> +static u32 *gen9_init_perctx_bb(struct intel_engine_cs *engine, u32 *batch)
>>  {
>> -	uint32_t index = wa_ctx_start(wa_ctx, *offset, CACHELINE_DWORDS);
>> -
>> -	wa_ctx_emit(batch, index, MI_BATCH_BUFFER_END);
>> +	*batch++ = MI_BATCH_BUFFER_END;
>>
>> -	return wa_ctx_end(wa_ctx, *offset = index, 1);
>> +	return batch;
>>  }
>
> Transformation looks reasonable, but I'd like to omit this per-ctx bb
> when empty.

Don't know if that is possible. It is always programming the offset at 
the moment so documentation digging is required.

>> +	/*
>> +	 * Emit the two workaround batch buffers, recording the offset from the
>> +	 * start of the workaround batch buffer object for each and their
>> +	 * respective sizes.
>> +	 */
>> +	for (i = 0; i < ARRAY_SIZE(wa_bb_f); i++) {
>> +		wa_bb[i]->offset = ALIGN(batch_ptr - batch, CACHELINE_DWORDS);
>> +		batch_ptr = wa_bb_f[i](engine, batch_ptr);
>> +		wa_bb[i]->size = batch_ptr - &batch[wa_bb[i]->offset];
>
> Which will break this pattern. At the least we could do
>
> if (!ww_bb_fn[i])
> 	continue;
>
> And then skip loading into the context image if size==0.
>
> I'll double check the mechanical aspects in a bit.

This should have been an RFC, just a mistake on my part for not marking 
it as such. It saves around 500-600 bytes AFAIR, but also I was thinking 
of adding a helper to emit the often used pipe control 
(gen8_emit_flush_render) which can bring more gains.

Regards,

Tvrtko
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