drm/ttm: rework pipelined eviction fence handling

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.

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Link: https://lore.kernel.org/r/20251121101315.3585-20-pierre-eric.pelloux-prayer@amd.com
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>

1 files changed, 21 insertions, 8 deletions

diff --git a/include/drm/ttm/ttm_resource.h b/include/drm/ttm/ttm_resource.h
index 68bf010d8b40..33e80f30b8b8 100644
--- a/include/drm/ttm/ttm_resource.h
+++ b/include/drm/ttm/ttm_resource.h
@@ -52,6 +52,15 @@ 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
  */
 enum ttm_lru_item_type {
@@ -181,8 +190,8 @@ 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.
@@ -196,12 +205,12 @@ 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.
@@ -422,8 +431,12 @@ 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);