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

[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.

> 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.

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?

Dave.