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drm/xe/bo: optimise TT population for DONTNEED BOs

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When a VRAM buffer object is marked as DONTNEED, like in Mesa, the
driver skips migrating its contents to system memory and instead purges
the backing store during eviction (via xe_ttm_bo_purge).

However, xe_evict_flags() still returns tt_placement (XE_PL_TT) for VRAM
BOs even if they were marked DONTNEED.  This causes
ttm_bo_handle_move_mem() to always call ttm_bo_populate() to allocate
destination system pages, only for those pages to be immediately freed
right after when xe_bo_move() calls xe_ttm_bo_purge().

Fix this by changing xe_evict_flags() to return sys_placement
(XE_PL_SYSTEM) for DONTNEED BOs.  This causes TTM to skip the population
step while still ensuring that TTM calls xe_bo_move(), so
xe_ttm_bo_purge() still triggers.

v2 (Thomas):
  - Add a comment to highlight why sys_placement over purge_placement.

Assisted-by: Copilot:gemini-3.1-pro-preview
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Maarten Lankhorst <dev@lankhorst.se>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260723161542.1220276-2-matthew.auld@intel.com
This commit is contained in:
Matthew Auld
2026-07-23 17:15:43 +01:00
parent a1e4e62ebc
commit 05649458e4
1 changed files with 12 additions and 0 deletions
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12
drivers/gpu/drm/xe/xe_bo.c
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@@ -341,6 +341,18 @@ static void xe_evict_flags(struct ttm_buffer_object *tbo,
return;
}
if (xe_bo_madv_is_dontneed(bo)) {
/*
* We can't use purge_placement here, since we need to trigger
* our own purge procedure at the start of xe_bo_move(), which
* would otherwise be skipped. At the same time we don't want
* ttm to then populate the tt with dst pages, before the move
* callback, hence use sys_placement here.
*/
*placement = sys_placement;
return;
}
/*
* For xe, sg bos that are evicted to system just triggers a
* rebind of the sg list upon subsequent validation to XE_PL_TT.