[Intel-gfx] [PATCH 24/66] drm/i915: Move aliasing_ppgtt

[Daniel Vetter]

On Mon, Jul 1, 2013 at 8:52 PM, Ben Widawsky <ben at bwidawsk.net> wrote:
>> One thing which looks a bit peculiar at the end is that struct
>> i915_hw_ppgtt is actually used as the real ppgtt object (since it
>> subclases i915_address space). My original plan was that we'll add a new
>> struct i915_ppgtt {
>>       struct i915_address_space base;
>>       struct i915_hw_ppgtt hw_ppgtt;
>> }
>>
>> To fit into your design the alising ppgtt pointer in dev_priv->gtt would
>> then only point at a hw_ppgtt struct, not the full deal with address space
>> and everything else around attached.
>>
>> Cheers, Daniel
>
> I don't think creating a struct i915_ppgtt is necessary or buys much. We
> can rename i915_hw_ppgtt to i915_ppgtt, and it accomplishes the same
> thing. Same for the i915_hw_context for that matter. I wanted to do any
> sort of renaming after the rest of the series.
>
> Can you explain why we'd want to keep the hw_ppgtt, and ppgtt with the
> track lists etc. distinct?

The difference is that for the aliasing ppgtt we don't really need a
address space object to track object offsets since those are identical
to the ggtt offsets. Hence we only need the hw part of the ppgtt, i.e.
writing/clearing ptes.

Full ppgtt otoh need the address space of course.

The above structure layout would allow that. It means that aliasing
ppgtt will stick out a bit like a sore thumb but imo that's ok since
it really _is_ a bit an odd thing. I've just noticed that you've
partially disabled the has_aliasing_ppgtt_mapping logic in your
branch, so I wonder a bit how aliasing ppgtt works there. But I guess
the simplest approach would be to simply keep that stuff around as-is,
with a few twists to wrestle it into the new world: Batches would be
executed as if they'd run in the global gtt (i.e. for eviction/binding
purposes) with the simple difference that we'd check whether the
global gtt has the aliasing_ppgtt pointer set up, and then write the
ptes there. Note that unconditionally writing both ppgtt and global
gtt ptes isn't too good, since in some virtualized enviroments
tracking those separately is a pretty substantial win apparently.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch