drivers: ethernet: eth_nxp_enet_qos: increase default buffer descriptors

Increased the default number of TX and RX buffer descriptors from 4 to
16. Since the current default buffer size (CONFIG_NET_BUF_DATA_SIZE) is
128, increasing the number of RX buffers is needed to be able to receive
at least one frame of a maximum size split between multiple buffers.

The default number of TX buffers was increased to match the number of RX
buffers and to be able to transmit large frames with many fragments.

Added build-time configuration validation to ensure that the combined
size of all RX buffers is sufficient to receive a maximum-sized Ethernet
frame.

Signed-off-by: Stanislav Poboril <stanislav.poboril@nxp.com>

Author: stanislav-poboril

drivers/ethernet/eth_nxp_enet_qos/Kconfig

@@ -36,17 +36,21 @@ config ETH_NXP_ENET_QOS_MAC_UNIQUE_MAC_ADDRESS
 
 config ETH_NXP_ENET_QOS_TX_BUFFER_DESCRIPTORS
 	int "Number of tx buffer descriptors"
-	default 4
+	default 16
 	range 4 1024
 	help
 	  Number of TX buffer descriptors.
 
 config ETH_NXP_ENET_QOS_RX_BUFFER_DESCRIPTORS
 	int "Number of rx buffer descriptors"
-	default 4
+	default 16
 	range 4 1024
 	help
 	  Number of RX buffer descriptors.
+	  Each descriptor references a buffer of a size of CONFIG_NET_BUF_DATA_SIZE
+	  (with the two least significant bits ignored), so set this to a value
+	  large enough so that at least one frame of a maximum size can be
+	  received into all buffers combined.
 
 config ETH_NXP_ENET_QOS_DMA_RESET_WAIT_TIME
 	int "Time in microseconds to wait for software reset"

drivers/ethernet/eth_nxp_enet_qos/eth_nxp_enet_qos_mac.c

@@ -24,6 +24,10 @@ LOG_MODULE_REGISTER(eth_nxp_enet_qos_mac, CONFIG_ETHERNET_LOG_LEVEL);
 #include "../eth.h"
 #include "nxp_enet_qos_priv.h"
 
+/* Verify configuration */
+BUILD_ASSERT((ENET_QOS_RX_BUFFER_SIZE * NUM_RX_BUFDESC) >= ENET_QOS_MAX_NORMAL_FRAME_LEN,
+	"ENET_QOS_RX_BUFFER_SIZE * NUM_RX_BUFDESC is not large enough to receive a full frame");
+
 static const uint32_t rx_desc_refresh_flags =
 	OWN_FLAG | RX_INTERRUPT_ON_COMPLETE_FLAG | BUF1_ADDR_VALID_FLAG;