[Intel-gfx] [PATCH] drm/i915: avoid wait_for_atomic() in non-atomic host2guc_action()

[Chris Wilson]

On Wed, Jul 06, 2016 at 03:30:11PM +0100, Dave Gordon wrote:
> Rather than using wait_for_atomic() when chacking for a response from
> the GuC, we can get the effect of a hybrid spin/sleep wait by breaking
> it into two stages. First, spin-wait for up to 10us to minimise latency
> for "quick" commands; then, if that times out, sleep-wait for up 10ms
> (the maximum allowed for a "slow" command).
> 
> Being able to do this depends on the recent patch
> 18f4b84 drm/i915: Use atomic waits for short non-atomic ones
> and is similar to the hybrid approach in
> 1758b90 drm/i915: Use a hybrid scheme for fast register waits
> (although we can't use that as-is, because that interface doesn't quite
> match what we need here).

Returning the status from wait_for_register would help for one other
callsite (gmbus iirc), not worth the conversion.

> Signed-off-by: Dave Gordon <david.s.gordon at intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

This entire sequence is under the FORCEWAKE_ALL (from inspection of
intel_uncore, I think you only need FORCEWAKE_BLITTER), you could use
I915_WRITE_FW / I915_READ_FW here - you lose both the spinlock and
auto-arming on each read, but you also lose the mmiotracing. (Though
realistically we should use the general mmiotracer and fix it if it
doesn't work for us.)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre