[PATCH 8/8] net: mrmac: increase the Rx BD ring

With only two Rx descriptors the ring is full as soon as two frames
arrive before the network stack drains them, and any further frame is
dropped by hardware until a descriptor is freed.

eth_rx() processes up to ETH_PACKETS_BATCH_RECV packets in one call, so
size the Rx ring from that constant. A full call can then be served
from the ring without hardware running out of descriptors, and the ring
depth follows if that constant ever changes.

Add static_assert() for the hardware minimum of two descriptors per
direction, so a smaller value fails the build instead of silently
making MRMAC drop packets.

This is headroom rather than a speed up. A TFTP transfer sends one
block at a time and waits for the ACK, so it keeps at most one packet
in flight and does not benefit from the deeper ring.

Signed-off-by: Padmarao Begari <[email protected]>
---
 drivers/net/xilinx_axi_mrmac.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/net/xilinx_axi_mrmac.h b/drivers/net/xilinx_axi_mrmac.h
index 2c5c83421e5..29f7c440238 100644
--- a/drivers/net/xilinx_axi_mrmac.h
+++ b/drivers/net/xilinx_axi_mrmac.h
@@ -11,14 +11,26 @@
 #ifndef __XILINX_AXI_MRMAC_H
 #define __XILINX_AXI_MRMAC_H
 
+#include <net.h>
+#include <linux/build_bug.h>
+
 #define MIN_PKT_SIZE	60
 
 /* MRMAC needs atleast two buffer descriptors for Tx/Rx to work.
  * Otherwise MRMAC will drop the packets. So, have atleast two Tx and
  * two Rx bd's.
+ *
+ * Tx keeps the minimum because send() waits for the transfer to
+ * complete, so a deeper Tx ring would never hold more than one frame.
+ * Rx matches the number of packets eth_rx() retires in one call, so a
+ * full batch can be taken from the ring without hardware dropping a
+ * frame for want of a free descriptor.
  */
 #define TX_DESC		2
-#define RX_DESC		2
+#define RX_DESC		ETH_PACKETS_BATCH_RECV
+
+static_assert(TX_DESC >= 2, "MRMAC needs at least two Tx descriptors");
+static_assert(RX_DESC >= 2, "MRMAC needs at least two Rx descriptors");
 
 /* MRMAC platform data structure */
 struct axi_mrmac_plat {
-- 
2.34.1