[Intel-gfx] [PATCH 51/59] drm/i915: Add *_ring_begin() to request allocation

[Daniel Vetter]

On Thu, Mar 19, 2015 at 12:30:56PM +0000, John.C.Harrison at Intel.com wrote:
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 6f198df..c7dcabd 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -2205,21 +2205,27 @@ int intel_ring_alloc_request_extras(struct drm_i915_gem_request *request)
>  	return 0;
>  }
>  
> -int intel_ring_reserved_space_reserve(struct intel_ringbuffer *ringbuf, int size)
> +int legacy_ring_reserved_space_reserve(struct drm_i915_gem_request *request)
>  {
> -	/* NB: Until request management is fully tidied up and the OLR is
> -	 * removed, there are too many ways for get false hits on this
> -	 * anti-recursion check! */
> -	/*WARN_ON(ringbuf->reserved_size);*/
> +	/*
> +	 * The first call merely notes the reserve request and is common for
> +	 * all back ends. The subsequent localised _begin() call actually
> +	 * ensures that the reservation is available. Without the begin, if
> +	 * the request creator immediately submitted the request without
> +	 * adding any commands to it then there might not actually be
> +	 * sufficient room for the submission commands.
> +	 */
> +	intel_ring_reserved_space_reserve(request->ringbuf, MIN_SPACE_FOR_ADD_REQUEST);
> +
> +	return intel_ring_begin(request, 0);

This feels a bit convoluted tbh, and would fall aparat if we start adding
sanity checks to _begin/advance functions. Can't we instead directly call
ring_wait_for_space? This forgoes the intel_wrap_ring_buffer call, but
otoh we just need to factor that into our estimates. Wrapping the ring for
the entire reservartion right away is
a) way too much - we only wrap individual ring_being calls anyway
b) not doing any good since all the intermediate intel_ring_emit calls
might very-well push us into a wrap anyway.

In the end we just need to increase our reservation with the biggest
intel_ring_begin we have in the add_request code - that's the worst-case
of ring space we might waste due to wrapping.

Same for the lrc path ofc.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch