[Intel-gfx] [PATCH] RFC: i915: Drop relocation support on Gen12+

[Chris Wilson]

Quoting Jason Ekstrand (2020-05-07 16:36:00)
> The Vulkan driver in Mesa for Intel hardware never uses relocations if
> it's running on a version of i915 that supports at least softpin which
> all versions of i915 supporting Gen12 do.  On the OpenGL side, Gen12 is
> only supported by iris which never uses relocations.  The older i965
> driver in Mesa does use relocations but it only supports Intel hardware
> through Gen11 and has been deprecated for all hardware Gen9+. The entire
> relocation UAPI and related infrastructure, therefore, doesn't have any
> open-source userspace consumer starting with Gen12.
> 
> Rejecting relocations starting with Gen12 has the benefit that we don't
> have to bother supporting it on platforms with local memory.  Given how
> much CPU touching of memory is required for relocations, not having to
> do so on platforms where not all memory is directly CPU-accessible
> carries significant advantages.

You are not supplying them, the kernel is not checking them [as they
don't exist], so there is no material benefit. The only question is
maintainability.

How confident are you that you will never use them and rewrite the
media-driver? The code exists, will be tested, and can just as easily
expire with the rest of execbuffer2.
-Chris