[Intel-gfx] [PATCH v3 5/5] drm/i915/execlists: Read the context-status HEAD from the HWSP

[Chris Wilson]

Quoting Chris Wilson (2017-07-13 11:27:06)
> The engine also provides a mirror of the CSB write pointer in the HWSP,
> but not of our read pointer. To take advantage of this we need to
> remember where we read up to on the last interrupt and continue off from
> there. This poses a problem following a reset, as we don't know where
> the hw will start writing from, and due to the use of power contexts we
> cannot perform that query during the reset itself. So we continue the
> current modus operandi of delaying the first read of the context-status
> read/write pointers until after the first interrupt. With this we should
> now have eliminated all uncached mmio reads in handling the
> context-status interrupt, though we still have the uncached mmio writes
> for submitting new work, and many uncached mmio reads in the global
> interrupt handler itself. Still a step in the right direction towards
> reducing our resubmit latency, although it appears lost in the noise!

It is also worth noting that Broxton seems to handle per-engine resets
better when using the HWSP than CSB mmio. Too early to say for sure,
needs a few days to be sure the error doesn't occur.
-Chris