ethernet: eth_sam_gmac: enable multiple TX pkt support for max throughput

This change enables GMAC_MULTIPLE_TX_PACKETS to allow multiple TX packets
to be pushed to the DMA engine, improving transmit performance. Previously,
older Zephyr versions (pre-v2.6.0) lacked thread-safe handling in net_pkt
and net_buf, making multiple TX enqueuing unreliable. Issue #32564
("net_buf reference count not protected") was fixed in v2.6.0, providing
proper thread safety. The changes have been tested on SAME54 Curiosity
Ultra with EVB-LAN8670-RMII and KSZ8061 PHY daughter card, and the
expected performance of 9.5Mbps at 10Mbps PHY link speed was observed.

Signed-off-by: Parthiban Veerasooran <[email protected]>

Author: ParthibanI17164

drivers/ethernet/eth_sam_gmac_priv.h

@@ -16,12 +16,8 @@
 #define ATMEL_OUI_B1 0x04
 #define ATMEL_OUI_B2 0x25
 
-/* This option enables support to push multiple packets to the DMA engine.
- * This currently doesn't work given the current version of net_pkt or
- * net_buf does not allowed access from multiple threads. This option is
- * therefore currently disabled.
- */
-#define GMAC_MULTIPLE_TX_PACKETS 0
+/* This option enables support to push multiple packets to the DMA engine. */
+#define GMAC_MULTIPLE_TX_PACKETS 1
 
 #define GMAC_MTU NET_ETH_MTU
 #define GMAC_FRAME_SIZE_MAX (GMAC_MTU + 18)