[PATCH 37/45] drm/ttm: add a helper to allocate a temp tt for copies.

[Daniel Vetter]

On Fri, Sep 25, 2020 at 9:39 AM Christian König
<christian.koenig at amd.com> wrote:
>
> Am 25.09.20 um 01:14 schrieb Dave Airlie:
> > On Thu, 24 Sep 2020 at 22:42, Christian König <christian.koenig at amd.com> wrote:
> >> Am 24.09.20 um 07:18 schrieb Dave Airlie:
> >>> From: Dave Airlie <airlied at redhat.com>
> >>>
> >>> All the accel moves do the same pattern here, provide a helper
> >> And exactly that pattern I want to get away from.
> > Currently this is just refactoring out the helper code in each driver, but I see
> > since it calls bo_mem_space we are probably moving a bit in the wrong direction.
>
> Exactly that's why I'm noting this.
>
> >
> >> See what happens if we (for example) have a VRAM -> SYSTEM move is the
> >> following:
> >>
> >> 1. TTM allocates a new ttm_resource object in the SYSTEM domain.
> >> 2. We call the driver to move from VRAM to SYSTEM.
> >> 3. Driver finds that it can't do this and calls TTM  to allocate GTT.
> >> 4. Since we are maybe out of GTT TTM evicts a different BO from GTT to
> >> SYSTEM and call driver again.
> >>
> >> This is a horrible ping/pong between driver/TTM/driver/TTM/driver and we
> >> should stop that immediately.
> >>
> >> My suggestion is that we rewrite how drivers call the ttm_bo_validate()
> >> function so that we can guarantee that this never happens.
> >>
> >> What do you think?
> > I think that is likely the next step I'd like to take after this
> > refactor, it's a lot bigger, and I'm not sure how it will look yet.
>
> Agree, yes. I have some ideas in mind for that, but not fully baked either.
>
> > Do we envision the driver calling validate in a loop but when it can't
> > find space it tells the driver and the driver does eviction and
> > recalls validate?
>
> Not in a loop, but more like in a chain.
>
> My plan is something like this:
> Instead of having "normal" and "busy" placement we have a flag in the
> context if evictions are allowed or not.
> The call to ttm_bo_validate are then replaced with two calls, first
> without evictions and if that didn't worked one with evictions.
>
> Then the normal validate sequence should look like this:
> 1. If a BO is in the SYSTEM (or SWAP domain) we validate it to GTT first
> with evictions=true.
> 2. If a BO should be in VRAM we then validate it to VRAM. If evictions
> are only allowed if the GEM flags say that GTT is not desired.

That solves the trouble when you move a bo into vram as part of
validate. But I'm not seeing how this solves the "need gtt mapping to
move something out of vram" problem.

Or should we instead move the entire eviction logic out from ttm into
drivers, building it up from helpers? Then drivers which need gtt for
moving stuff out of vram can do that right away. Also, this would
allow us to implement very fancy eviction algorithms like all the
nonsense we're doing in i915 for gtt handling on gen2/3 (but I really
hope that never ever becomes a thing again in future gpus, so this is
maybe more a what-if kind of thing). Not sure how that would look
like, maybe a special validate function which takes a ttm_resource the
driver already found (through evicting stuff or whatever) and then ttm
just does the move and book-keeping and everything. And drivers would
at first only call validate without allowing any eviction. Ofc anyone
without special needs could use the standard eviction function that
validate already has.
-Daniel

> For special BOs, like amdgpus GDS, GWS and OA domain or VMWGFX special
> domains that will obviously look a bit different.
>
> Christian.
>
> >
> > Dave.
>
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch