[PATCH net-next v5 08/14] net: ethernet: cortina: Rotate free queue page allocation

[Linus Walleij <linusw@kernel.org>]

The free queue metadata bitmap currently searches for a free slot from
zero for every page allocation. Under sustained traffic this repeatedly
scans occupied low-numbered slots.

Remember the position after the most recently allocated slot and wrap the
bitmap search at its end. Reset the cursor when the bitmap is released so
the two remain consistent across failed setup and teardown. This keeps
allocation cost distributed across the metadata table.

Assisted-by: LLM
Signed-off-by: Linus Walleij <linusw@kernel.org>
---
 drivers/net/ethernet/cortina/gemini.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/cortina/gemini.c b/drivers/net/ethernet/cortina/gemini.c
index faab1bb865ab..1d71c5066457 100644
--- a/drivers/net/ethernet/cortina/gemini.c
+++ b/drivers/net/ethernet/cortina/gemini.c
@@ -174,6 +174,7 @@ struct gemini_ethernet {
 	struct xarray	freeq_mappings;
 	unsigned int	num_freeq_pages;
 	unsigned long	*freeq_page_bitmap;
+	unsigned int	freeq_page_cursor;
 	spinlock_t	freeq_lock; /* Locks queue from reentrance */
 };
 
@@ -745,12 +746,20 @@ static int geth_freeq_alloc_slot(struct gemini_ethernet *geth)
 
 	lockdep_assert_held(&geth->freeq_lock);
 
-	slot = find_first_zero_bit(geth->freeq_page_bitmap,
-				   geth->num_freeq_pages);
-	if (slot == geth->num_freeq_pages)
-		return -ENOSPC;
+	slot = find_next_zero_bit(geth->freeq_page_bitmap,
+				  geth->num_freeq_pages,
+				  geth->freeq_page_cursor);
+	if (slot == geth->num_freeq_pages) {
+		slot = find_first_zero_bit(geth->freeq_page_bitmap,
+					   geth->freeq_page_cursor);
+		if (slot == geth->freeq_page_cursor)
+			return -ENOSPC;
+	}
 
 	__set_bit(slot, geth->freeq_page_bitmap);
+	geth->freeq_page_cursor = slot + 1;
+	if (geth->freeq_page_cursor == geth->num_freeq_pages)
+		geth->freeq_page_cursor = 0;
 
 	return slot;
 }
@@ -1113,6 +1122,7 @@ static int geth_setup_freeq(struct gemini_ethernet *geth)
 	if (!geth->freeq_page_bitmap)
 		goto err_freeq_pages;
 	geth->num_freeq_pages = page_slots;
+	geth->freeq_page_cursor = 0;
 
 	/* Match the software pointer to the retained hardware pointer. */
 	rptr = GET_RPTR(geth->base + GLOBAL_SWFQ_RWPTR_REG);
@@ -1144,6 +1154,7 @@ static int geth_setup_freeq(struct gemini_ethernet *geth)
 	kfree(geth->freeq_pages);
 	geth->freeq_pages = NULL;
 	geth->num_freeq_pages = 0;
+	geth->freeq_page_cursor = 0;
 err_freeq:
 	dma_free_coherent(geth->dev,
 			  sizeof(*geth->freeq_ring) << geth->freeq_order,
@@ -1172,6 +1183,7 @@ static void geth_cleanup_freeq(struct gemini_ethernet *geth)
 	kfree(geth->freeq_pages);
 	geth->freeq_pages = NULL;
 	geth->num_freeq_pages = 0;
+	geth->freeq_page_cursor = 0;
 
 	dma_free_coherent(geth->dev,
 			  sizeof(*geth->freeq_ring) << geth->freeq_order,

-- 
2.56.0