[PATCH v2 03/33] drm/ttm: Record sub-optimal page order allocations in ttm_tt

[Matthew Brost]

When __ttm_pool_alloc() fails to allocate a chunk at the device's
beneficial order and falls back to a smaller order, the object ends up
backed by a sub-optimal set of pages. Nothing currently records this, so
a driver has no way to know an object would benefit from being re-backed
with beneficial-order pages later.

Add TTM_TT_FLAG_BENEFICIAL_ORDER_FAILED to struct ttm_tt page_flags,
cleared at the start of every fresh allocation (in both ttm_pool_alloc()
and ttm_pool_restore_and_alloc()). Set it in __ttm_pool_alloc() when an
allocation at exactly the beneficial order fails before dropping to a
lower order.

Drivers can use this hint to queue the buffer object for later
defragmentation.

Cc: Carlos Santa <carlos.santa@xxxxxxxxx>
Cc: Ryan Neph <ryanneph@xxxxxxxxxx>
Cc: Christian Koenig <christian.koenig@xxxxxxx>
Cc: Huang Rui <ray.huang@xxxxxxx>
Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Simona Vetter <simona@xxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Assisted-by: GitHub_Copilot:claude-opus-4.8
Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
---
 drivers/gpu/drm/ttm/ttm_pool.c | 13 +++++++++++++
 include/drm/ttm/ttm_tt.h       | 24 +++++++++++++++++++++++-
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
index d34592d4dbc7..370d991c9882 100644
--- a/drivers/gpu/drm/ttm/ttm_pool.c
+++ b/drivers/gpu/drm/ttm/ttm_pool.c
@@ -796,6 +796,7 @@ static int __ttm_pool_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
 			    struct ttm_pool_alloc_state *alloc,
 			    struct ttm_pool_tt_restore *restore)
 {
+	const unsigned int beneficial_order = ttm_pool_beneficial_order(pool);
 	enum ttm_caching page_caching;
 	gfp_t gfp_flags = GFP_USER;
 	pgoff_t caching_divide;
@@ -846,6 +847,16 @@ static int __ttm_pool_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
 		/* If that fails, lower the order if possible and retry. */
 		if (!p) {
 			if (order) {
+				/*
+				 * Failing to allocate at the device's beneficial
+				 * order means we are about to back this object
+				 * with a sub-optimal (smaller order) set of
+				 * pages. Record it so the driver can later try to
+				 * defragment the object back to beneficial order.
+				 */
+				if (beneficial_order && order == beneficial_order)
+					tt->page_flags |=
+						TTM_TT_FLAG_BENEFICIAL_ORDER_FAILED;
 				--order;
 				page_caching = tt->caching;
 				allow_pools = true;
@@ -910,6 +921,7 @@ int ttm_pool_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
 	if (WARN_ON(ttm_tt_is_backed_up(tt)))
 		return -EINVAL;
 
+	tt->page_flags &= ~TTM_TT_FLAG_BENEFICIAL_ORDER_FAILED;
 	ttm_pool_alloc_state_init(tt, &alloc);
 
 	return __ttm_pool_alloc(pool, tt, ctx, &alloc, NULL);
@@ -942,6 +954,7 @@ int ttm_pool_restore_and_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
 	if (!restore) {
 		gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
 
+		tt->page_flags &= ~TTM_TT_FLAG_BENEFICIAL_ORDER_FAILED;
 		ttm_pool_alloc_state_init(tt, &alloc);
 		if (ctx->gfp_retry_mayfail)
 			gfp |= __GFP_RETRY_MAYFAIL;
diff --git a/include/drm/ttm/ttm_tt.h b/include/drm/ttm/ttm_tt.h
index 406437ad674b..ce7533677d77 100644
--- a/include/drm/ttm/ttm_tt.h
+++ b/include/drm/ttm/ttm_tt.h
@@ -90,6 +90,14 @@ struct ttm_tt {
 	 * TTM_TT_FLAG_BACKED_UP: TTM internal only. This is set if the
 	 * struct ttm_tt has been (possibly partially) backed up.
 	 *
+	 * TTM_TT_FLAG_BENEFICIAL_ORDER_FAILED: Set by the TTM pool allocator
+	 * when at least one chunk that could have been allocated at the pool's
+	 * beneficial order had to fall back to a smaller order. This indicates
+	 * that a sub-optimal set of pages was chosen for this object, and can
+	 * be used by drivers to queue the buffer object for later
+	 * defragmentation. Drivers should access this via the
+	 * ttm_tt_is_beneficial_order_failed() helper.
+	 *
 	 * TTM_TT_FLAG_PRIV_POPULATED: TTM internal only. DO NOT USE. This is
 	 * set by TTM after ttm_tt_populate() has successfully returned, and is
 	 * then unset when TTM calls ttm_tt_unpopulate().
@@ -101,8 +109,9 @@ struct ttm_tt {
 #define TTM_TT_FLAG_EXTERNAL_MAPPABLE	BIT(3)
 #define TTM_TT_FLAG_DECRYPTED		BIT(4)
 #define TTM_TT_FLAG_BACKED_UP	        BIT(5)
+#define TTM_TT_FLAG_BENEFICIAL_ORDER_FAILED	BIT(6)
 
-#define TTM_TT_FLAG_PRIV_POPULATED	BIT(6)
+#define TTM_TT_FLAG_PRIV_POPULATED	BIT(7)
 	uint32_t page_flags;
 	/** @num_pages: Number of pages in the page array. */
 	uint32_t num_pages;
@@ -179,6 +188,19 @@ static inline void ttm_tt_clear_backed_up(struct ttm_tt *tt)
 	tt->page_flags &= ~TTM_TT_FLAG_BACKED_UP;
 }
 
+/**
+ * ttm_tt_is_beneficial_order_failed() - Whether the tt is backed at a
+ * sub-optimal page order
+ * @tt: The struct ttm_tt.
+ *
+ * Return: true if the pool allocator had to fall back below the pool's
+ * beneficial order when backing this tt, false otherwise.
+ */
+static inline bool ttm_tt_is_beneficial_order_failed(const struct ttm_tt *tt)
+{
+	return tt->page_flags & TTM_TT_FLAG_BENEFICIAL_ORDER_FAILED;
+}
+
 /**
  * ttm_tt_create
  *
-- 
2.34.1