[PATCH net v4 4/5] net: macb: single dma_alloc_coherent() for DMA descriptors

["Théo Lebrun" <theo.lebrun@bootlin.com>]

Move from 2*NUM_QUEUES dma_alloc_coherent() for DMA descriptor rings to
2 calls overall.

Issue is with how all queues share the same register for configuring the
upper 32-bits of Tx/Rx descriptor rings. Taking Tx, notice how TBQPH
does *not* depend on the queue index:

	#define GEM_TBQP(hw_q)		(0x0440 + ((hw_q) << 2))
	#define GEM_TBQPH(hw_q)		(0x04C8)

	queue_writel(queue, TBQP, lower_32_bits(queue->tx_ring_dma));
	#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
	if (bp->hw_dma_cap & HW_DMA_CAP_64B)
		queue_writel(queue, TBQPH, upper_32_bits(queue->tx_ring_dma));
	#endif

To maximise our chances of getting valid DMA addresses, we do a single
dma_alloc_coherent() across queues. This improves the odds because
alloc_pages() guarantees natural alignment. Other codepaths (IOMMU or
dev/arch dma_map_ops) don't give high enough guarantees
(even page-aligned isn't enough).

Two consideration:

 - dma_alloc_coherent() gives us page alignment. Here we remove this
   constraint meaning each queue's ring won't be page-aligned anymore.

 - This can save some tiny amounts of memory. Fewer allocations means
   (1) less overhead (constant cost per alloc) and (2) less wasted bytes
   due to alignment constraints.

   Example for (2): 4 queues, default ring size (512), 64-bit DMA
   descriptors, 16K pages:
    - Before: 8 allocs of 8K, each rounded to 16K => 64K wasted.
    - After:  2 allocs of 32K => 0K wasted.

Fixes: 02c958dd3446 ("net/macb: add TX multiqueue support for gem")
Reviewed-by: Sean Anderson <sean.anderson@linux.dev>
Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
---
 drivers/net/ethernet/cadence/macb_main.c | 80 ++++++++++++++++----------------
 1 file changed, 41 insertions(+), 39 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index d413e8bd4977187fd73f7cc48268baf933aab051..7f31f264a6d342ea01e2f61944b12c9b9a3fe66e 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -2474,32 +2474,30 @@ static unsigned int macb_rx_ring_size_per_queue(struct macb *bp)
 
 static void macb_free_consistent(struct macb *bp)
 {
+	struct device *dev = &bp->pdev->dev;
 	struct macb_queue *queue;
 	unsigned int q;
+	size_t size;
 
 	if (bp->rx_ring_tieoff) {
-		dma_free_coherent(&bp->pdev->dev, macb_dma_desc_get_size(bp),
+		dma_free_coherent(dev, macb_dma_desc_get_size(bp),
 				  bp->rx_ring_tieoff, bp->rx_ring_tieoff_dma);
 		bp->rx_ring_tieoff = NULL;
 	}
 
 	bp->macbgem_ops.mog_free_rx_buffers(bp);
 
+	size = bp->num_queues * macb_tx_ring_size_per_queue(bp);
+	dma_free_coherent(dev, size, bp->queues[0].tx_ring, bp->queues[0].tx_ring_dma);
+
+	size = bp->num_queues * macb_rx_ring_size_per_queue(bp);
+	dma_free_coherent(dev, size, bp->queues[0].rx_ring, bp->queues[0].rx_ring_dma);
+
 	for (q = 0, queue = bp->queues; q < bp->num_queues; ++q, ++queue) {
 		kfree(queue->tx_skb);
 		queue->tx_skb = NULL;
-		if (queue->tx_ring) {
-			dma_free_coherent(&bp->pdev->dev,
-					  macb_tx_ring_size_per_queue(bp),
-					  queue->tx_ring, queue->tx_ring_dma);
-			queue->tx_ring = NULL;
-		}
-		if (queue->rx_ring) {
-			dma_free_coherent(&bp->pdev->dev,
-					  macb_rx_ring_size_per_queue(bp),
-					  queue->rx_ring, queue->rx_ring_dma);
-			queue->rx_ring = NULL;
-		}
+		queue->tx_ring = NULL;
+		queue->rx_ring = NULL;
 	}
 }
 
@@ -2541,41 +2539,45 @@ static int macb_alloc_rx_buffers(struct macb *bp)
 
 static int macb_alloc_consistent(struct macb *bp)
 {
+	struct device *dev = &bp->pdev->dev;
+	dma_addr_t tx_dma, rx_dma;
 	struct macb_queue *queue;
 	unsigned int q;
-	u32 upper;
-	int size;
+	void *tx, *rx;
+	size_t size;
+
+	/*
+	 * Upper 32-bits of Tx/Rx DMA descriptor for each queues much match!
+	 * We cannot enforce this guarantee, the best we can do is do a single
+	 * allocation and hope it will land into alloc_pages() that guarantees
+	 * natural alignment of physical addresses.
+	 */
+
+	size = bp->num_queues * macb_tx_ring_size_per_queue(bp);
+	tx = dma_alloc_coherent(dev, size, &tx_dma, GFP_KERNEL);
+	if (!tx || upper_32_bits(tx_dma) != upper_32_bits(tx_dma + size - 1))
+		goto out_err;
+	netdev_dbg(bp->dev, "Allocated %zu bytes for %u TX rings at %08lx (mapped %p)\n",
+		   size, bp->num_queues, (unsigned long)tx_dma, tx);
+
+	size = bp->num_queues * macb_rx_ring_size_per_queue(bp);
+	rx = dma_alloc_coherent(dev, size, &rx_dma, GFP_KERNEL);
+	if (!rx || upper_32_bits(rx_dma) != upper_32_bits(rx_dma + size - 1))
+		goto out_err;
+	netdev_dbg(bp->dev, "Allocated %zu bytes for %u RX rings at %08lx (mapped %p)\n",
+		   size, bp->num_queues, (unsigned long)rx_dma, rx);
 
 	for (q = 0, queue = bp->queues; q < bp->num_queues; ++q, ++queue) {
-		size = macb_tx_ring_size_per_queue(bp);
-		queue->tx_ring = dma_alloc_coherent(&bp->pdev->dev, size,
-						    &queue->tx_ring_dma,
-						    GFP_KERNEL);
-		upper = upper_32_bits(queue->tx_ring_dma);
-		if (!queue->tx_ring ||
-		    upper != upper_32_bits(bp->queues[0].tx_ring_dma))
-			goto out_err;
-		netdev_dbg(bp->dev,
-			   "Allocated TX ring for queue %u of %d bytes at %08lx (mapped %p)\n",
-			   q, size, (unsigned long)queue->tx_ring_dma,
-			   queue->tx_ring);
+		queue->tx_ring = tx + macb_tx_ring_size_per_queue(bp) * q;
+		queue->tx_ring_dma = tx_dma + macb_tx_ring_size_per_queue(bp) * q;
+
+		queue->rx_ring = rx + macb_rx_ring_size_per_queue(bp) * q;
+		queue->rx_ring_dma = rx_dma + macb_rx_ring_size_per_queue(bp) * q;
 
 		size = bp->tx_ring_size * sizeof(struct macb_tx_skb);
 		queue->tx_skb = kmalloc(size, GFP_KERNEL);
 		if (!queue->tx_skb)
 			goto out_err;
-
-		size = macb_rx_ring_size_per_queue(bp);
-		queue->rx_ring = dma_alloc_coherent(&bp->pdev->dev, size,
-						    &queue->rx_ring_dma,
-						    GFP_KERNEL);
-		upper = upper_32_bits(queue->rx_ring_dma);
-		if (!queue->rx_ring ||
-		    upper != upper_32_bits(bp->queues[0].rx_ring_dma))
-			goto out_err;
-		netdev_dbg(bp->dev,
-			   "Allocated RX ring of %d bytes at %08lx (mapped %p)\n",
-			   size, (unsigned long)queue->rx_ring_dma, queue->rx_ring);
 	}
 	if (bp->macbgem_ops.mog_alloc_rx_buffers(bp))
 		goto out_err;

-- 
2.50.1