tsnep: Replace modulo operation with mask

TX/RX ring size is static and power of 2 to enable compiler to optimize
modulo operation to mask operation. Make this optimization already in
the code and don't rely on the compiler.

CPU utilisation during high packet rate has not changed. So no
performance improvement has been measured. But it is best practice to
prevent modulo operations.
Suggested-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/engleder/tsnep.h

@@ -18,6 +18,7 @@
 #define TSNEP "tsnep"
 
 #define TSNEP_RING_SIZE 256
+#define TSNEP_RING_MASK (TSNEP_RING_SIZE - 1)
 #define TSNEP_RING_RX_REFILL 16
 #define TSNEP_RING_RX_REUSE (TSNEP_RING_SIZE - TSNEP_RING_SIZE / 4)
 #define TSNEP_RING_ENTRIES_PER_PAGE (PAGE_SIZE / TSNEP_DESC_SIZE)

drivers/net/ethernet/engleder/tsnep_main.c

@@ -292,7 +292,7 @@ static int tsnep_tx_ring_init(struct tsnep_tx *tx)
 	}
 	for (i = 0; i < TSNEP_RING_SIZE; i++) {
 		entry = &tx->entry[i];
-		next_entry = &tx->entry[(i + 1) % TSNEP_RING_SIZE];
+		next_entry = &tx->entry[(i + 1) & TSNEP_RING_MASK];
 		entry->desc->next = __cpu_to_le64(next_entry->desc_dma);
 	}
 
@@ -381,7 +381,7 @@ static int tsnep_tx_map(struct sk_buff *skb, struct tsnep_tx *tx, int count)
 	int i;
 
 	for (i = 0; i < count; i++) {
-		entry = &tx->entry[(tx->write + i) % TSNEP_RING_SIZE];
+		entry = &tx->entry[(tx->write + i) & TSNEP_RING_MASK];
 
 		if (!i) {
 			len = skb_headlen(skb);
@@ -419,7 +419,7 @@ static int tsnep_tx_unmap(struct tsnep_tx *tx, int index, int count)
 	int i;
 
 	for (i = 0; i < count; i++) {
-		entry = &tx->entry[(index + i) % TSNEP_RING_SIZE];
+		entry = &tx->entry[(index + i) & TSNEP_RING_MASK];
 
 		if (entry->len) {
 			if (entry->type & TSNEP_TX_TYPE_SKB)
@@ -481,9 +481,9 @@ static netdev_tx_t tsnep_xmit_frame_ring(struct sk_buff *skb,
 		skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
 
 	for (i = 0; i < count; i++)
-		tsnep_tx_activate(tx, (tx->write + i) % TSNEP_RING_SIZE, length,
+		tsnep_tx_activate(tx, (tx->write + i) & TSNEP_RING_MASK, length,
 				  i == count - 1);
-	tx->write = (tx->write + count) % TSNEP_RING_SIZE;
+	tx->write = (tx->write + count) & TSNEP_RING_MASK;
 
 	skb_tx_timestamp(skb);
 
@@ -516,7 +516,7 @@ static int tsnep_xdp_tx_map(struct xdp_frame *xdpf, struct tsnep_tx *tx,
 	frag = NULL;
 	len = xdpf->len;
 	for (i = 0; i < count; i++) {
-		entry = &tx->entry[(tx->write + i) % TSNEP_RING_SIZE];
+		entry = &tx->entry[(tx->write + i) & TSNEP_RING_MASK];
 		if (type & TSNEP_TX_TYPE_XDP_NDO) {
 			data = unlikely(frag) ? skb_frag_address(frag) :
 						xdpf->data;
@@ -589,9 +589,9 @@ static bool tsnep_xdp_xmit_frame_ring(struct xdp_frame *xdpf,
 	length = retval;
 
 	for (i = 0; i < count; i++)
-		tsnep_tx_activate(tx, (tx->write + i) % TSNEP_RING_SIZE, length,
+		tsnep_tx_activate(tx, (tx->write + i) & TSNEP_RING_MASK, length,
 				  i == count - 1);
-	tx->write = (tx->write + count) % TSNEP_RING_SIZE;
+	tx->write = (tx->write + count) & TSNEP_RING_MASK;
 
 	/* descriptor properties shall be valid before hardware is notified */
 	dma_wmb();
@@ -691,7 +691,7 @@ static bool tsnep_tx_poll(struct tsnep_tx *tx, int napi_budget)
 		/* xdpf is union with skb */
 		entry->skb = NULL;
 
-		tx->read = (tx->read + count) % TSNEP_RING_SIZE;
+		tx->read = (tx->read + count) & TSNEP_RING_MASK;
 
 		tx->packets++;
 		tx->bytes += length + ETH_FCS_LEN;
@@ -839,7 +839,7 @@ static int tsnep_rx_ring_init(struct tsnep_rx *rx)
 
 	for (i = 0; i < TSNEP_RING_SIZE; i++) {
 		entry = &rx->entry[i];
-		next_entry = &rx->entry[(i + 1) % TSNEP_RING_SIZE];
+		next_entry = &rx->entry[(i + 1) & TSNEP_RING_MASK];
 		entry->desc->next = __cpu_to_le64(next_entry->desc_dma);
 	}
 
@@ -925,7 +925,7 @@ static int tsnep_rx_refill(struct tsnep_rx *rx, int count, bool reuse)
 	int retval;
 
 	for (i = 0; i < count && !alloc_failed; i++) {
-		index = (rx->write + i) % TSNEP_RING_SIZE;
+		index = (rx->write + i) & TSNEP_RING_MASK;
 
 		retval = tsnep_rx_alloc_buffer(rx, index);
 		if (unlikely(retval)) {
@@ -945,7 +945,7 @@ static int tsnep_rx_refill(struct tsnep_rx *rx, int count, bool reuse)
 	}
 
 	if (enable) {
-		rx->write = (rx->write + i) % TSNEP_RING_SIZE;
+		rx->write = (rx->write + i) & TSNEP_RING_MASK;
 
 		/* descriptor properties shall be valid before hardware is
 		 * notified
@@ -1090,7 +1090,7 @@ static int tsnep_rx_poll(struct tsnep_rx *rx, struct napi_struct *napi,
 				 * empty RX ring, thus buffer cannot be used for
 				 * RX processing
 				 */
-				rx->read = (rx->read + 1) % TSNEP_RING_SIZE;
+				rx->read = (rx->read + 1) & TSNEP_RING_MASK;
 				desc_available++;
 
 				rx->dropped++;
@@ -1117,7 +1117,7 @@ static int tsnep_rx_poll(struct tsnep_rx *rx, struct napi_struct *napi,
 		 */
 		length -= TSNEP_RX_INLINE_METADATA_SIZE;
 
-		rx->read = (rx->read + 1) % TSNEP_RING_SIZE;
+		rx->read = (rx->read + 1) & TSNEP_RING_MASK;
 		desc_available++;
 
 		if (prog) {