[PATCH v2 02/20] drm/ttm: rework pipelined eviction fence handling

Pierre-Eric Pelloux-Prayer
2025-11-13T16:07:55Z

Until now ttm stored a single pipelined eviction fence which means
drivers had to use a single entity for these evictions.

To lift this requirement, this commit allows up to 8 entities to
be used.

Ideally a dma_resv object would have been used as a container of
the eviction fences, but the locking rules makes it complex.
dma_resv all have the same ww_class, which means "Attempting to
lock more mutexes after ww_acquire_done." is an error.

One alternative considered was to introduced a 2nd ww_class for
specific resv to hold a single "transient" lock (= the resv lock
would only be held for a short period, without taking any other
locks).

The other option, is to statically reserve a fence array, and
extend the existing code to deal with N fences, instead of 1.

The driver is still responsible to reserve the correct number
of fence slots.

---
v2:
- simplified code
- dropped n_fences
- name changes
---

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
---
drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 8 ++--
.../gpu/drm/ttm/tests/ttm_bo_validate_test.c | 11 +++--
drivers/gpu/drm/ttm/tests/ttm_resource_test.c | 5 +-
drivers/gpu/drm/ttm/ttm_bo.c | 47 ++++++++++---------
drivers/gpu/drm/ttm/ttm_bo_util.c | 38 ++++++++++++---
drivers/gpu/drm/ttm/ttm_resource.c | 31 +++++++-----
include/drm/ttm/ttm_resource.h | 29 ++++++++----
7 files changed, 109 insertions(+), 60 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
index 326476089db3..3b46a24a8c48 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
@@ -2156,7 +2156,7 @@ void amdgpu_ttm_set_buffer_funcs_status(struct amdgpu_device *adev, bool enable)
{
struct ttm_resource_manager *man = ttm_manager_type(&adev->mman.bdev, TTM_PL_VRAM);
uint64_t size;
- int r;
+ int r, i;

if (!adev->mman.initialized || amdgpu_in_reset(adev) ||
adev->mman.buffer_funcs_enabled == enable || adev->gmc.is_app_apu)
@@ -2190,8 +2190,10 @@ void amdgpu_ttm_set_buffer_funcs_status(struct amdgpu_device *adev, bool enable)
} else {
drm_sched_entity_destroy(&adev->mman.high_pr);
drm_sched_entity_destroy(&adev->mman.low_pr);
- dma_fence_put(man->move);
- man->move = NULL;
+ for (i = 0; i < TTM_NUM_MOVE_FENCES; i++) {
+ dma_fence_put(man->eviction_fences[i]);
+ man->eviction_fences[i] = NULL;
+ }
}

/* this just adjusts TTM size idea, which sets lpfn to the correct value */
diff --git a/drivers/gpu/drm/ttm/tests/ttm_bo_validate_test.c b/drivers/gpu/drm/ttm/tests/ttm_bo_validate_test.c
index 3148f5d3dbd6..8f71906c4238 100644
--- a/drivers/gpu/drm/ttm/tests/ttm_bo_validate_test.c
+++ b/drivers/gpu/drm/ttm/tests/ttm_bo_validate_test.c
@@ -651,7 +651,7 @@ static void ttm_bo_validate_move_fence_signaled(struct kunit *test)
int err;

man = ttm_manager_type(priv->ttm_dev, mem_type);
- man->move = dma_fence_get_stub();
+ man->eviction_fences[0] = dma_fence_get_stub();

bo = ttm_bo_kunit_init(test, test->priv, size, NULL);
bo->type = bo_type;
@@ -668,7 +668,7 @@ static void ttm_bo_validate_move_fence_signaled(struct kunit *test)
KUNIT_EXPECT_EQ(test, ctx.bytes_moved, size);

ttm_bo_put(bo);
- dma_fence_put(man->move);
+ dma_fence_put(man->eviction_fences[0]);
}

static const struct ttm_bo_validate_test_case ttm_bo_validate_wait_cases[] = {
@@ -732,9 +732,9 @@ static void ttm_bo_validate_move_fence_not_signaled(struct kunit *test)

spin_lock_init(&fence_lock);
man = ttm_manager_type(priv->ttm_dev, fst_mem);
- man->move = alloc_mock_fence(test);
+ man->eviction_fences[0] = alloc_mock_fence(test);

- task = kthread_create(threaded_fence_signal, man->move, "move-fence-signal");
+ task = kthread_create(threaded_fence_signal, man->eviction_fences[0], "move-fence-signal");
if (IS_ERR(task))
KUNIT_FAIL(test, "Couldn't create move fence signal task\n");

@@ -742,7 +742,8 @@ static void ttm_bo_validate_move_fence_not_signaled(struct kunit *test)
err = ttm_bo_validate(bo, placement_val, &ctx_val);
dma_resv_unlock(bo->base.resv);

- dma_fence_wait_timeout(man->move, false, MAX_SCHEDULE_TIMEOUT);
+ dma_fence_wait_timeout(man->eviction_fences[0], false, MAX_SCHEDULE_TIMEOUT);
+ man->eviction_fences[0] = NULL;

KUNIT_EXPECT_EQ(test, err, 0);
KUNIT_EXPECT_EQ(test, ctx_val.bytes_moved, size);
diff --git a/drivers/gpu/drm/ttm/tests/ttm_resource_test.c b/drivers/gpu/drm/ttm/tests/ttm_resource_test.c
index e6ea2bd01f07..c0e4e35e0442 100644
--- a/drivers/gpu/drm/ttm/tests/ttm_resource_test.c
+++ b/drivers/gpu/drm/ttm/tests/ttm_resource_test.c
@@ -207,6 +207,7 @@ static void ttm_resource_manager_init_basic(struct kunit *test)
struct ttm_resource_test_priv *priv = test->priv;
struct ttm_resource_manager *man;
size_t size = SZ_16K;
+ int i;

man = kunit_kzalloc(test, sizeof(*man), GFP_KERNEL);
KUNIT_ASSERT_NOT_NULL(test, man);
@@ -216,8 +217,8 @@ static void ttm_resource_manager_init_basic(struct kunit *test)
KUNIT_ASSERT_PTR_EQ(test, man->bdev, priv->devs->ttm_dev);
KUNIT_ASSERT_EQ(test, man->size, size);
KUNIT_ASSERT_EQ(test, man->usage, 0);
- KUNIT_ASSERT_NULL(test, man->move);
- KUNIT_ASSERT_NOT_NULL(test, &man->move_lock);
+ for (i = 0; i < TTM_NUM_MOVE_FENCES; i++)
+ KUNIT_ASSERT_NULL(test, man->eviction_fences[i]);

for (int i = 0; i < TTM_MAX_BO_PRIORITY; ++i)
KUNIT_ASSERT_TRUE(test, list_empty(&man->lru[i]));
diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index f4d9e68b21e7..0b3732ed6f6c 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -658,34 +658,35 @@ void ttm_bo_unpin(struct ttm_buffer_object *bo)
EXPORT_SYMBOL(ttm_bo_unpin);

/*
- * Add the last move fence to the BO as kernel dependency and reserve a new
- * fence slot.
+ * Add the pipelined eviction fencesto the BO as kernel dependency and reserve new
+ * fence slots.
*/
-static int ttm_bo_add_move_fence(struct ttm_buffer_object *bo,
- struct ttm_resource_manager *man,
- bool no_wait_gpu)
+static int ttm_bo_add_pipelined_eviction_fences(struct ttm_buffer_object *bo,
+ struct ttm_resource_manager *man,
+ bool no_wait_gpu)
{
struct dma_fence *fence;
- int ret;
+ int i;

- spin_lock(&man->move_lock);
- fence = dma_fence_get(man->move);
- spin_unlock(&man->move_lock);
+ spin_lock(&man->eviction_lock);
+ for (i = 0; i < TTM_NUM_MOVE_FENCES; i++) {
+ fence = man->eviction_fences[i];
+ if (!fence)
+ continue;

- if (!fence)
- return 0;
-
- if (no_wait_gpu) {
- ret = dma_fence_is_signaled(fence) ? 0 : -EBUSY;
- dma_fence_put(fence);
- return ret;
+ if (no_wait_gpu) {
+ if (!dma_fence_is_signaled(fence)) {
+ spin_unlock(&man->eviction_lock);
+ return -EBUSY;
+ }
+ } else {
+ dma_resv_add_fence(bo->base.resv, fence, DMA_RESV_USAGE_KERNEL);
+ }
}
+ spin_unlock(&man->eviction_lock);

- dma_resv_add_fence(bo->base.resv, fence, DMA_RESV_USAGE_KERNEL);
-
- ret = dma_resv_reserve_fences(bo->base.resv, 1);
- dma_fence_put(fence);
- return ret;
+ /* TODO: this call should be removed. */
+ return dma_resv_reserve_fences(bo->base.resv, 1);
}

/**
@@ -718,7 +719,7 @@ static int ttm_bo_alloc_resource(struct ttm_buffer_object *bo,
int i, ret;

ticket = dma_resv_locking_ctx(bo->base.resv);
- ret = dma_resv_reserve_fences(bo->base.resv, 1);
+ ret = dma_resv_reserve_fences(bo->base.resv, TTM_NUM_MOVE_FENCES);
if (unlikely(ret))
return ret;

@@ -757,7 +758,7 @@ static int ttm_bo_alloc_resource(struct ttm_buffer_object *bo,
return ret;
}

- ret = ttm_bo_add_move_fence(bo, man, ctx->no_wait_gpu);
+ ret = ttm_bo_add_pipelined_eviction_fences(bo, man, ctx->no_wait_gpu);
if (unlikely(ret)) {
ttm_resource_free(bo, res);
if (ret == -EBUSY)
diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
index acbbca9d5c92..2ff35d55e462 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_util.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
@@ -258,7 +258,7 @@ static int ttm_buffer_object_transfer(struct ttm_buffer_object *bo,
ret = dma_resv_trylock(&fbo->base.base._resv);
WARN_ON(!ret);

- ret = dma_resv_reserve_fences(&fbo->base.base._resv, 1);
+ ret = dma_resv_reserve_fences(&fbo->base.base._resv, TTM_NUM_MOVE_FENCES);
if (ret) {
dma_resv_unlock(&fbo->base.base._resv);
kfree(fbo);
@@ -646,20 +646,44 @@ static void ttm_bo_move_pipeline_evict(struct ttm_buffer_object *bo,
{
struct ttm_device *bdev = bo->bdev;
struct ttm_resource_manager *from;
+ struct dma_fence *tmp;
+ int i;

from = ttm_manager_type(bdev, bo->resource->mem_type);

/**
* BO doesn't have a TTM we need to bind/unbind. Just remember
- * this eviction and free up the allocation
+ * this eviction and free up the allocation.
+ * The fence will be saved in the first free slot or in the slot
+ * already used to store a fence from the same context. Since
+ * drivers can't use more than TTM_NUM_MOVE_FENCES contexts for
+ * evictions we should always find a slot to use.
*/
- spin_lock(&from->move_lock);
- if (!from->move || dma_fence_is_later(fence, from->move)) {
- dma_fence_put(from->move);
- from->move = dma_fence_get(fence);
+ spin_lock(&from->eviction_lock);
+ for (i = 0; i < TTM_NUM_MOVE_FENCES; i++) {
+ tmp = from->eviction_fences[i];
+ if (!tmp)
+ break;
+ if (fence->context != tmp->context)
+ continue;
+ if (dma_fence_is_later(fence, tmp)) {
+ dma_fence_put(tmp);
+ break;
+ }
+ goto unlock;
+ }
+ if (i < TTM_NUM_MOVE_FENCES) {
+ from->eviction_fences[i] = dma_fence_get(fence);
+ } else {
+ WARN(1, "not enough fence slots for all fence contexts");
+ spin_unlock(&from->eviction_lock);
+ dma_fence_wait(fence, false);
+ goto end;
}
- spin_unlock(&from->move_lock);

+unlock:
+ spin_unlock(&from->eviction_lock);
+end:
ttm_resource_free(bo, &bo->resource);
}

diff --git a/drivers/gpu/drm/ttm/ttm_resource.c b/drivers/gpu/drm/ttm/ttm_resource.c
index e2c82ad07eb4..62c34cafa387 100644
--- a/drivers/gpu/drm/ttm/ttm_resource.c
+++ b/drivers/gpu/drm/ttm/ttm_resource.c
@@ -523,14 +523,15 @@ void ttm_resource_manager_init(struct ttm_resource_manager *man,
{
unsigned i;

- spin_lock_init(&man->move_lock);
man->bdev = bdev;
man->size = size;
man->usage = 0;

for (i = 0; i < TTM_MAX_BO_PRIORITY; ++i)
INIT_LIST_HEAD(&man->lru[i]);
- man->move = NULL;
+ spin_lock_init(&man->eviction_lock);
+ for (i = 0; i < TTM_NUM_MOVE_FENCES; i++)
+ man->eviction_fences[i] = NULL;
}
EXPORT_SYMBOL(ttm_resource_manager_init);

@@ -551,7 +552,7 @@ int ttm_resource_manager_evict_all(struct ttm_device *bdev,
.no_wait_gpu = false,
};
struct dma_fence *fence;
- int ret;
+ int ret, i;

do {
ret = ttm_bo_evict_first(bdev, man, &ctx);
@@ -561,18 +562,24 @@ int ttm_resource_manager_evict_all(struct ttm_device *bdev,
if (ret && ret != -ENOENT)
return ret;

- spin_lock(&man->move_lock);
- fence = dma_fence_get(man->move);
- spin_unlock(&man->move_lock);
+ ret = 0;

- if (fence) {
- ret = dma_fence_wait(fence, false);
- dma_fence_put(fence);
- if (ret)
- return ret;
+ spin_lock(&man->eviction_lock);
+ for (i = 0; i < TTM_NUM_MOVE_FENCES; i++) {
+ fence = man->eviction_fences[i];
+ if (fence && !dma_fence_is_signaled(fence)) {
+ dma_fence_get(fence);
+ spin_unlock(&man->eviction_lock);
+ ret = dma_fence_wait(fence, false);
+ dma_fence_put(fence);
+ if (ret)
+ return ret;
+ spin_lock(&man->eviction_lock);
+ }
}
+ spin_unlock(&man->eviction_lock);

- return 0;
+ return ret;
}
EXPORT_SYMBOL(ttm_resource_manager_evict_all);

diff --git a/include/drm/ttm/ttm_resource.h b/include/drm/ttm/ttm_resource.h
index f49daa504c36..50e6added509 100644
--- a/include/drm/ttm/ttm_resource.h
+++ b/include/drm/ttm/ttm_resource.h
@@ -50,6 +50,15 @@ struct io_mapping;
struct sg_table;
struct scatterlist;

+/**
+ * define TTM_NUM_MOVE_FENCES - How many entities can be used for evictions
+ *
+ * Pipelined evictions can be spread on multiple entities. This
+ * is the max number of entities that can be used by the driver
+ * for that purpose.
+ */
+#define TTM_NUM_MOVE_FENCES 8
+
/**
* enum ttm_lru_item_type - enumerate ttm_lru_item subclasses
*/
@@ -180,8 +189,8 @@ struct ttm_resource_manager_func {
* @size: Size of the managed region.
* @bdev: ttm device this manager belongs to
* @func: structure pointer implementing the range manager. See above
- * @move_lock: lock for move fence
- * @move: The fence of the last pipelined move operation.
+ * @eviction_lock: lock for eviction fences
+ * @eviction_fences: The fences of the last pipelined move operation.
* @lru: The lru list for this memory type.
*
* This structure is used to identify and manage memory types for a device.
@@ -195,12 +204,12 @@ struct ttm_resource_manager {
struct ttm_device *bdev;
uint64_t size;
const struct ttm_resource_manager_func *func;
- spinlock_t move_lock;

- /*
- * Protected by @move_lock.
+ /* This is very similar to a dma_resv object, but locking rules make
+ * it difficult to use one in this context.
*/
- struct dma_fence *move;
+ spinlock_t eviction_lock;
+ struct dma_fence *eviction_fences[TTM_NUM_MOVE_FENCES];

/*
* Protected by the bdev->lru_lock.
@@ -421,8 +430,12 @@ static inline bool ttm_resource_manager_used(struct ttm_resource_manager *man)
static inline void
ttm_resource_manager_cleanup(struct ttm_resource_manager *man)
{
- dma_fence_put(man->move);
- man->move = NULL;
+ int i;
+
+ for (i = 0; i < TTM_NUM_MOVE_FENCES; i++) {
+ dma_fence_put(man->eviction_fences[i]);
+ man->eviction_fences[i] = NULL;
+ }
}

void ttm_lru_bulk_move_init(struct ttm_lru_bulk_move *bulk);
--
2.43.0