On 31/08/2022 13:35, Christian König wrote:
Am 31.08.22 um 14:06 schrieb Matthew Auld:
On 31/08/2022 12:03, Christian König wrote:
Am 31.08.22 um 12:37 schrieb Matthew Auld:
[SNIP]
That hopefully just leaves i915_ttm_shrink(), which is swapping
out shmem ttm_tt and is calling ttm_bo_validate() with empty
placements to force the pipeline-gutting path, which importantly
unpopulates the ttm_tt for us (since ttm_tt_unpopulate is not
exported it seems). But AFAICT it looks like that will now also
nuke the bo->resource, instead of just leaving it in system
memory. My assumption is that when later calling
ttm_bo_validate(), it will just do the bo_move_null() in
i915_ttm_move(), instead of re-populating the ttm_tt and then
potentially copying it back to local-memory?
Well you do ttm_bo_validate() with something like GTT domain, don't
you? This should result in re-populating the tt object, but I'm not
100% sure if that really works as expected.
AFAIK for domains we either have system memory (which uses ttm_tt
and might be shmem underneath) or local-memory. But perhaps i915 is
doing something wrong here, or abusing TTM in some way. I'm not sure
tbh.
Anyway, I think we have two cases here:
- We have some system memory only object. After doing
i915_ttm_shrink(), bo->resource is now NULL. We then call
ttm_bo_validate() at some later point, but here we don't need to
copy anything, but it also looks like ttm_bo_handle_move_mem() won't
populate the ttm_tt or us either, since mem_type == TTM_PL_SYSTEM.
It looks like i915_ttm_move() was taking care of this, but now we
just call ttm_bo_move_null().
- We have a local-memory only object, which was evicted to shmem,
and then swapped out by the shrinker like above. The bo->resource is
NULL. However this time when calling ttm_bo_validate() we need to
actually do a copy in i915_ttm_move(), as well as re-populate the
ttm_tt. i915_ttm_move() was taking care of this, but now we just
call ttm_bo_move_null().
Perhaps i915 is doing something wrong in the above two cases?
Mhm, as far as I can see that should still work.
See previously you should got a transition from SYSTEM->GTT in
i915_ttm_move() to re-create your backing store. Not you get
NULL->SYSTEM which is handled by ttm_bo_move_null() and then
SYSTEM->GTT.
What is GTT here in TTM world? Also I'm not seeing where there is this
SYSTEM->GTT transition? Maybe I'm blind. Just to be clear, i915 is
only calling ttm_bo_validate() once when acquiring the pages, and we
don't call it again, unless it was evicted (and potentially swapped out).
Well GTT means TTM_PL_TT.
And calling it only once is perfectly fine, TTM will internally see that
we need two hops to reach TTM_PL_TT and so does the NULL->SYSTEM
transition and then SYSTEM->TT.
Ah interesting, so that's what the multi-hop thing does. But AFAICT i915
is not using either TTM_PL_TT or -EMULTIHOP.
Also what is the difference between TTM_PL_TT and TM_PL_SYSTEM? When
should you use one over the other?
As far as I can see that should work like it did before.
Christian.
If you just validated to SYSTEM memory before I think the tt object
wouldn't have been populated either.
Regards,
Christian.
Thanks,
Christian.
I've been considering to replacing the ttm_bo_type with a bunch
of behavior flags for a bo. I'm hoping that this will clean
things up a bit.
Regards,
Christian.
caching = i915_ttm_select_tt_caching(obj);
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c
b/drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c
index 9a7e50534b84bb..c420d1ab605b6f 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm_move.c
@@ -560,7 +560,7 @@ int i915_ttm_move(struct ttm_buffer_object
*bo, bool evict,
bool clear;
int ret;
- if (GEM_WARN_ON(!obj)) {
+ if (GEM_WARN_ON(!obj) || !bo->resource) {
ttm_bo_move_null(bo, dst_mem);
return 0;
}
