From: Matthew Auld matthew.auld@intel.com

[ Upstream commit edb1745fc618ba8ef63a45ce3ae60de1bdf29231 ]

Since the dma-resv is shared we don't need to reserve and add a fence slot fence twice, plus no need to loop through the dependencies.

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: Jonathan Cavitt jonathan.cavitt@intel.com Reviewed-by: Thomas Hellström thomas.hellstrom@linux.intel.com Link: https://lore.kernel.org/r/20250829164715.720735-2-matthew.auld@intel.com Signed-off-by: Sasha Levin sashal@kernel.org ---

LLM Generated explanations, may be completely bogus:

YES

Explanation

- What it fixes - Removes redundant dma-resv operations when a backup BO shares the same reservation object as the original BO, preventing the same fence from being reserved/added twice to the same `dma_resv`. - Avoids scanning the same dependency set twice when source and destination BOs share the same `dma_resv`.

- Why the change is correct - The backup object is created to share the parent’s reservation object, so a single reserve/add is sufficient: - The backup BO is initialized with the parent’s resv: `drivers/gpu/drm/xe/xe_bo.c:1309` (`xe_bo_init_locked(..., bo->ttm.base.resv, ...)`), ensuring `bo->ttm.base.resv == backup->ttm.base.resv`. - The patch adds an explicit invariant check to document and enforce this: `drivers/gpu/drm/xe/xe_bo.c:1225` (`xe_assert(xe, bo->ttm.base.resv == backup->ttm.base.resv)`). - With shared `dma_resv`, adding the same fence twice is at best redundant (wasting fence slots and memory) and at worst error-prone. Reserving fence slots only once and adding the fence once is the correct behavior.

- Specific code changes and effects - Evict path (GPU migration copy case): - Before: reserves and adds fence on both `bo->ttm.base.resv` and `backup->ttm.base.resv`. - After: reserves and adds exactly once, guarded by the shared-resv assertion. - See single reserve and add: `drivers/gpu/drm/xe/xe_bo.c:1226` (reserve) and `drivers/gpu/drm/xe/xe_bo.c:1237` (add fence). This is the core fix; the removed second reserve/add on the backup is the redundant part eliminated. - Restore path (migration copy back): - Same simplification: reserve once, add once on the shared `dma_resv`. - See single reserve and add: `drivers/gpu/drm/xe/xe_bo.c:1375` (reserve) and `drivers/gpu/drm/xe/xe_bo.c:1387` (add fence). - Dependency handling in migrate: - Before: added deps for both src and dst based only on `src_bo != dst_bo`. - After: only add dst deps if the resv objects differ, avoiding double-walking the same `dma_resv`. - See updated condition: `drivers/gpu/drm/xe/xe_migrate.c:932` (`src_bo->ttm.base.resv != dst_bo->ttm.base.resv`).

- User-visible impact without the patch - Duplicate `dma_resv_add_fence()` calls on the same reservation object can: - Consume extra shared-fence slots and memory. - Inflate dependency lists, causing unnecessary scheduler waits and overhead. - Increase failure likelihood of `dma_resv_reserve_fences()` under memory pressure. - These paths are exercised during suspend/resume flows of pinned VRAM BOs (evict/restore), so reliability and performance in power transitions can be affected.

- Scope and risk - Small, focused changes localized to the Intel Xe driver migration/evict/restore paths: - Files: `drivers/gpu/drm/xe/xe_bo.c`, `drivers/gpu/drm/xe/xe_migrate.c`. - No API changes or architectural refactors; logic strictly reduces redundant operations. - The `xe_assert` acts as a safety net to catch unexpected non-shared `resv` usage; normal runtime behavior is unchanged when the invariant holds. - The CPU copy fallback paths are untouched.

- Stable backport considerations - This is a clear correctness and robustness fix, not a feature. - Low regression risk if the stable branch also creates the backup BO with the parent’s `dma_resv` (as shown by the use of `xe_bo_init_locked(..., bo->ttm.base.resv, ...)` in `drivers/gpu/drm/xe/xe_bo.c:1309`). - If a stable branch diverges and the backup BO does not share the resv, this patch would need adjustment (i.e., keep dual reserve/add in that case). The added `xe_assert` helps surface such mismatches during testing.

Conclusion: This commit fixes a real bug (duplicate fence reserve/add and duplicate dependency scanning on a shared `dma_resv`) with a minimal, well-scoped change. It aligns with stable rules (important bugfix, low risk, contained), so it should be backported.

drivers/gpu/drm/xe/xe_bo.c | 13 +------------ drivers/gpu/drm/xe/xe_migrate.c | 2 +- 2 files changed, 2 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c index d07e23eb1a54d..5a61441d68af5 100644 --- a/drivers/gpu/drm/xe/xe_bo.c +++ b/drivers/gpu/drm/xe/xe_bo.c @@ -1242,14 +1242,11 @@ int xe_bo_evict_pinned(struct xe_bo *bo) else migrate = mem_type_to_migrate(xe, bo->ttm.resource->mem_type);

+ xe_assert(xe, bo->ttm.base.resv == backup->ttm.base.resv); ret = dma_resv_reserve_fences(bo->ttm.base.resv, 1); if (ret) goto out_backup;

- ret = dma_resv_reserve_fences(backup->ttm.base.resv, 1); - if (ret) - goto out_backup; - fence = xe_migrate_copy(migrate, bo, backup, bo->ttm.resource, backup->ttm.resource, false); if (IS_ERR(fence)) { @@ -1259,8 +1256,6 @@ int xe_bo_evict_pinned(struct xe_bo *bo)

dma_resv_add_fence(bo->ttm.base.resv, fence, DMA_RESV_USAGE_KERNEL); - dma_resv_add_fence(backup->ttm.base.resv, fence, - DMA_RESV_USAGE_KERNEL); dma_fence_put(fence); } else { ret = xe_bo_vmap(backup); @@ -1338,10 +1333,6 @@ int xe_bo_restore_pinned(struct xe_bo *bo) if (ret) goto out_unlock_bo;

- ret = dma_resv_reserve_fences(backup->ttm.base.resv, 1); - if (ret) - goto out_unlock_bo; - fence = xe_migrate_copy(migrate, backup, bo, backup->ttm.resource, bo->ttm.resource, false); @@ -1352,8 +1343,6 @@ int xe_bo_restore_pinned(struct xe_bo *bo)

dma_resv_add_fence(bo->ttm.base.resv, fence, DMA_RESV_USAGE_KERNEL); - dma_resv_add_fence(backup->ttm.base.resv, fence, - DMA_RESV_USAGE_KERNEL); dma_fence_put(fence); } else { ret = xe_bo_vmap(backup); diff --git a/drivers/gpu/drm/xe/xe_migrate.c b/drivers/gpu/drm/xe/xe_migrate.c index 2a627ed64b8f8..ba9b8590eccb2 100644 --- a/drivers/gpu/drm/xe/xe_migrate.c +++ b/drivers/gpu/drm/xe/xe_migrate.c @@ -901,7 +901,7 @@ struct dma_fence *xe_migrate_copy(struct xe_migrate *m, if (!fence) { err = xe_sched_job_add_deps(job, src_bo->ttm.base.resv, DMA_RESV_USAGE_BOOKKEEP); - if (!err && src_bo != dst_bo) + if (!err && src_bo->ttm.base.resv != dst_bo->ttm.base.resv) err = xe_sched_job_add_deps(job, dst_bo->ttm.base.resv, DMA_RESV_USAGE_BOOKKEEP); if (err)