On ke, 2017-02-08 at 18:04 +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
>
> This removes the usage of intel_ring_emit in favour of
> directly writing to the ring buffer.
>
> intel_ring_emit was preventing the compiler for optimising
> fetch and increment of the current ring buffer pointer and
> therefore generating very verbose code for every write.
>
> It had no useful purpose since all ringbuffer operations
> are started and ended with intel_ring_begin and
> intel_ring_advance respectively, with no bail out in the
> middle possible, so it is fine to increment the tail in
> intel_ring_begin and let the code manage the pointer
> itself.
>
> Useless instruction removal amounts to approximately
> two and half kilobytes of saved text on my build.
>
> Not sure if this has any measurable performance
> implications but executing a ton of useless instructions
> on fast paths cannot be good.
>
> Patch is not fully polished, but it compiles and runs
> on Gen9 at least.
>
> v2:
> * Change return from intel_ring_begin to error pointer by
> popular demand.
> * Move tail increment to intel_ring_advance to enable some
> error checking.
>
> v3:
> * Move tail advance back into intel_ring_begin.
> * Rebase and tidy.
>
> v4:
> * Complete rebase after a few months since v3.
>
> v5:
> * Remove unecessary cast and fix !debug compile. (Chris Wilson)
>
> v6:
> * Make intel_ring_offset take request as well.
> * Fix recording of request postfix plus a sprinkle of asserts.
> (Chris Wilson)
>
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
<SNIP>
> @@ -617,99 +616,92 @@ mi_set_context(struct drm_i915_gem_request *req, u32 hw_flags)
> if (INTEL_GEN(dev_priv) >= 7)
> len += 2 + (num_rings ? 4*num_rings + 6 : 0);
>
> - ret = intel_ring_begin(req, len);
> - if (ret)
> - return ret;
> + out = intel_ring_begin(req, len);
> + if (IS_ERR(out))
> + return PTR_ERR(out);
>
> /* WaProgramMiArbOnOffAroundMiSetContext:ivb,vlv,hsw,bdw,chv */
> if (INTEL_GEN(dev_priv) >= 7) {
> - intel_ring_emit(ring, MI_ARB_ON_OFF | MI_ARB_DISABLE);
> + *out++ = MI_ARB_ON_OFF | MI_ARB_DISABLE;
I expressed my concern in the previous iteration of this series months
ago, and here goes again; Lets try to keep the writes easily greppable.
So intel_ring_emit (or better name) could remain as a wrapper
#define (something something)_emit(x, y) *(x)++ = (y)
Or, we make the name distinctive, "*ring++" would work for that.
Regards, Joonas
--
Joonas Lahtinen
Open Source Technology Center
Intel Corporation
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