[PATCH] drm/i915: Allow evicting to use the requested placement

[David Gow]

In a78a8da5 ("drm/ttm: replace busy placement with flags v6"), the old
system of having a separate placement list (for placements which should
be used without eviction) and a 'busy' placement list (for placements
which should be attempted if eviction is required) was replaced with a
new one where placements could be marked 'FALLBACK' (to be attempted if
eviction is required) or 'DESIRED' (to be attempted first, but not if
eviction is required).

i915 had always included the requested placement in the list of
'busy' placements: i.e., the placement could be used either if eviction
is required or not. But when the new system was put in place, the
requested (first) placement was marked 'DESIRED', so would never be used
if eviction became necessary. While a bug in the original commit
prevented this flag from working, when this was fixed in
4a0e7b3c ("drm/i915: fix applying placement flag"), it caused long hangs
on DG2 systems with small BAR.

Don't mark the requested placement DESIRED (or FALLBACK), allowing it to
be used in both situations. This matches the old behaviour, and resolves
the hangs.

Thanks to Justin Brewer for bisecting the issue.

Fixes: a78a8da51b36 ("drm/ttm: replace busy placement with flags v6")
Fixes: 4a0e7b3c3753 ("drm/i915: fix applying placement flag")
Link: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/11255
Signed-off-by: David Gow <david at davidgow.net>
---

I'm not 100% sure I understand exactly what went wrong here: I'm pretty
sure the patch is correct, but let me know if the commit description is
way off base.

My system works much better with this applied, but it's possible that it
could work better still with further changes: the buddy allocator is
still chewing up a lot of the CPU, and there are still cases where this
notably affects performance (though, in my experience, these are now
'hitches' rather than multi-minute hangs).

Cheers,
-- David

---
 drivers/gpu/drm/i915/gem/i915_gem_ttm.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
index e6f177183c0f..fb848fd8ba15 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
@@ -165,7 +165,6 @@ i915_ttm_placement_from_obj(const struct drm_i915_gem_object *obj,
 	i915_ttm_place_from_region(num_allowed ? obj->mm.placements[0] :
 				   obj->mm.region, &places[0], obj->bo_offset,
 				   obj->base.size, flags);
-	places[0].flags |= TTM_PL_FLAG_DESIRED;
 
 	/* Cache this on object? */
 	for (i = 0; i < num_allowed; ++i) {
-- 
2.45.2