[Intel-gfx] [PATCH v2 4/5] drm/i915/execlists: Read the context-status buffer from the HWSP

[Chris Wilson]

Quoting Wang, Zhi A (2017-07-13 10:36:14)
> Sorry, my bad for missing it as I only saw it used on CHV before your patch because of some HW issue.
> 
> Kinda of interested in the reason why HWSP CSB gains people's interest now? :P. Anyway, it's good move to reduce the MMIO trap here.

We've been chasing the elimination of mmio reads here for the last few
years, since the buffer is tantalising laid out inside a cacheline and
bspec says it was designed to be read in a single cacheline load.
Previously I had an alias for the CSB using ioremap_cache(), which
sort-of-worked (bxt was being a little unhappy) but still required a
clflush. Imagine then the surprise and delight that the information we
wanted was in the HWSP ever since execlists was a twinkle in the
architect's eye (back in gm45!).
-Chris