[Intel-gfx] [PATCH] drm/i915: Add interface to reserve fence registers for vGPU

[Chris Wilson]

Quoting changbin.du at intel.com (2017-08-30 09:54:21)
> This patch added two new api to the fence management code:
>  - i915_reserve_one_fence() will try to find a free fence from fence_list
>    and force-remove vma if need.
>  - i915_giveback_reserved_fence() reclaim a reserved fence after vGPU has
>    finished.

Symmetry: reserve_fence, unreserve_fence.

We need a safeguard here so that the host is able to always able allocate
to allocate a fence for the display engine. (That requirement should be
quite soft for modern hw, nevertheless it should be in the design to
prevent overuse from leading to an unusable system.)
-Chris