[Intel-gfx] [PATCH] drm/i915: Engine relative MMIO

[Chris Wilson]

Quoting John.C.Harrison at Intel.com (2019-03-30 00:10:45)
> From: John Harrison <John.C.Harrison at Intel.com>
> 
> With virtual engines, it is no longer possible to know which specific
> physical engine a given request will be executed on at the time that
> request is generated. This means that the request itself must be engine
> agnostic - any direct register writes must be relative to the engine
> and not absolute addresses.
> 
> The LRI command has support for engine relative addressing. However,
> the mechanism is not transparent to the driver. The scheme for Gen11
> (MI_LRI_ADD_CS_MMIO_START) requires the LRI address to have no
> absolute engine base component. The hardware then adds on the correct
> engine offset at execution time.
> 
> Due to the non-trivial and differing schemes on different hardware, it
> is not possible to simply update the code that creates the LRI
> commands to set a remap flag and let the hardware get on with it.
> Instead, this patch adds function wrappers for generating the LRI
> command itself and then for constructing the correct address to use
> with the LRI.
> 
> v2: Fix build break in GVT. Remove flags parameter [review feedback
> from Chris W].

I'm still asking why are we "changing" instructions that we know are tied
to an engine? The instruction is the same, it just gained an extra bit
to denote relative mmio offset.
-Chris