drivers: nrf_wifi: Implement TX zero-copy feature

This uses the network packet as is without the need for a copy all the
way till the packet is handed over to RPU.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>

drivers/wifi/nrf_wifi/Kconfig.nrfwifi

@@ -828,4 +828,25 @@ config NRF_WIFI_COEX_DISABLE_PRIORITY_WINDOW_FOR_SCAN
 	help
 	  Enable this configuration to disable priority window for scan
 	  in the case of coexistence with Short Range radio.
+
+if NETWORKING
+config NRF_WIFI_ZERO_COPY_TX
+	bool "Zero copy Transmit path [EXPERIMENTAL]"
+	select NET_L2_ETHERNET_RESERVE_HEADER
+	select EXPERIMENTAL
+	# 54L has lower RAM
+	default y if SOC_SERIES_NRF54LX
+	help
+	  Enable this configuration to use zero copy Transmit path.
+	  The driver will use the network buffer directly for transmission
+	  without copying the data to the driver's buffer. This reduces the
+	  driver heap memory usage without much impact on the performance.
+
+	  The application should configure the network buffers to ensure that
+	  the whole packet fits in a single buffer, else the driver will fallback
+	  to the normal copy path, but the memory requirements would still match
+	  to the zero copy path and may be sub-optimal for the normal copy path.
+
+endif # NETWORKING
+
 endif # WIFI_NRF70

drivers/wifi/nrf_wifi/src/net_if.c

@@ -1002,6 +1002,11 @@ int nrf_wifi_if_get_config_zep(const struct device *dev,
 					 ETHERNET_CHECKSUM_SUPPORT_TCP |
 					 ETHERNET_CHECKSUM_SUPPORT_UDP;
 	}
+#endif
+#ifdef CONFIG_NRF_WIFI_ZERO_COPY_TX
+	if (type == ETHERNET_CONFIG_TYPE_EXTRA_TX_PKT_HEADROOM) {
+		config->extra_tx_pkt_headroom = NRF_WIFI_EXTRA_TX_HEADROOM;
+	}
 #endif
 	ret = 0;
 unlock:

modules/nrf_wifi/os/shim.c

@@ -268,6 +268,9 @@ struct nwb {
 	void (*cleanup_cb)();
 	unsigned char priority;
 	bool chksum_done;
+#ifdef CONFIG_NRF_WIFI_ZERO_COPY_TX
+	struct net_pkt *pkt;
+#endif
 };
 
 static void *zep_shim_nbuf_alloc(unsigned int size)
@@ -301,6 +304,12 @@ static void zep_shim_nbuf_free(void *nbuf)
 	if (!nbuf) {
 		return;
 	}
+#ifdef CONFIG_NRF_WIFI_ZERO_COPY_TX
+	if (((struct nwb *)nbuf)->pkt) {
+		net_pkt_unref(((struct nwb *)nbuf)->pkt);
+		((struct nwb *)nbuf)->pkt = NULL;
+	}
+#endif /* CONFIG_NRF_WIFI_ZERO_COPY_TX */
 
 	zep_shim_data_mem_free(((struct nwb *)nbuf)->priv);
 	zep_shim_data_mem_free(nbuf);
@@ -387,12 +396,62 @@ static void zep_shim_nbuf_set_chksum_done(void *nbuf, unsigned char chksum_done)
 #include <zephyr/net/ethernet.h>
 #include <zephyr/net/net_core.h>
 
+#ifdef CONFIG_NRF_WIFI_ZERO_COPY_TX
+void *net_pkt_to_nbuf_zc(struct net_pkt *pkt)
+{
+	struct nwb *nbuff;
+
+	if (!pkt || !pkt->buffer) {
+		LOG_DBG("Invalid packet, dropping");
+		return NULL;
+	}
+
+	/* Check if packet has more than one fragment */
+	if (pkt->buffer->frags) {
+		LOG_ERR("Zero-copy only supports single buffer packets");
+		return NULL;
+	}
+
+	nbuff = zep_shim_nbuf_alloc(NRF_WIFI_EXTRA_TX_HEADROOM); /* Just for headers */
+	if (!nbuff) {
+		return NULL;
+	}
+
+	zep_shim_nbuf_headroom_res(nbuff, NRF_WIFI_EXTRA_TX_HEADROOM);
+
+	/* Zero-copy: point to the single data buffer */
+	/* TODO: Use API for proper cursor access? */
+	nbuff->data = pkt->buffer->data;
+	nbuff->len = pkt->buffer->len;
+
+	nbuff->priority = net_pkt_priority(pkt);
+	nbuff->chksum_done = (bool)net_pkt_is_chksum_done(pkt);
+
+	nbuff->pkt = pkt;
+	/* Ref the packet so that it is not freed */
+	net_pkt_ref(pkt);
+
+	return nbuff;
+}
+#endif /* CONFIG_NRF_WIFI_ZERO_COPY_TX */
+
 void *net_pkt_to_nbuf(struct net_pkt *pkt)
 {
 	struct nwb *nbuff;
 	unsigned char *data;
 	unsigned int len;
 
+	if (!pkt) {
+		return NULL;
+	}
+
+#ifdef CONFIG_NRF_WIFI_ZERO_COPY_TX
+	/* For zero-copy, check if packet has single buffer */
+	if (pkt->buffer && !pkt->buffer->frags) {
+		return net_pkt_to_nbuf_zc(pkt);
+	}
+#endif /* CONFIG_NRF_WIFI_ZERO_COPY_TX */
+
 	len = net_pkt_get_len(pkt);
 
 	nbuff = zep_shim_nbuf_alloc(len + 100);

modules/nrf_wifi/os/shim.h

@@ -16,6 +16,8 @@
 #include <zephyr/drivers/gpio.h>
 #include <zephyr/net/net_pkt.h>
 
+#define NRF_WIFI_EXTRA_TX_HEADROOM 100
+
 /**
  * struct zep_shim_bus_qspi_priv - Structure to hold context information for the Linux OS
  *                        shim.