net: ethernet: Allow drivers to reserve net_pkt headroom

Add new Ethernet driver config option,
ETHERNET_CONFIG_TYPE_EXTRA_TX_PKT_HEADROOM, which allows Ethernet
drivers to inform L2 about the extra net_pkt headroom they need to be
allocated.
This is only supported when CONFIG_NET_L2_ETHERNET_RESERVE_HEADER is
enabled, so that it's possible to fit entire packet into a single
net_buf, which is needed for zero-copy transmission.

(cherry picked from commit 78c3996)

Original-Signed-off-by: Robert Lubos <[email protected]>
GitOrigin-RevId: 78c3996
Cr-Build-Id: 8724909411128659441
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8724909411128659441
Copybot-Job-Name: zephyr-main-copybot-downstream
Change-Id: I9f53462110aae6f48b567a4ae139bd5cc774b2bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/6196364
Tested-by: ChromeOS Prod (Robot) <[email protected]>
Commit-Queue: ChromeOS Prod (Robot) <[email protected]>
Bot-Commit: ChromeOS Prod (Robot) <[email protected]>

Author: Robert Lubos

include/zephyr/net/ethernet.h

@@ -234,7 +234,8 @@ enum ethernet_config_type {
 	ETHERNET_CONFIG_TYPE_T1S_PARAM,
 	ETHERNET_CONFIG_TYPE_TXINJECTION_MODE,
 	ETHERNET_CONFIG_TYPE_RX_CHECKSUM_SUPPORT,
-	ETHERNET_CONFIG_TYPE_TX_CHECKSUM_SUPPORT
+	ETHERNET_CONFIG_TYPE_TX_CHECKSUM_SUPPORT,
+	ETHERNET_CONFIG_TYPE_EXTRA_TX_PKT_HEADROOM,
 };
 
 enum ethernet_qav_param_type {
@@ -529,6 +530,8 @@ struct ethernet_config {
 		enum ethernet_checksum_support chksum_support;
 
 		struct ethernet_filter filter;
+
+		uint16_t extra_tx_pkt_headroom;
 	};
 };
 

subsys/net/l2/ethernet/ethernet.c

@@ -862,8 +862,15 @@ static int ethernet_l2_alloc(struct net_if *iface, struct net_pkt *pkt,
 			     size_t size, enum net_ip_protocol proto,
 			     k_timeout_t timeout)
 {
-	return net_pkt_alloc_buffer_with_reserve(pkt, size,
-						 get_reserve_ll_header_size(iface),
+	size_t reserve = get_reserve_ll_header_size(iface);
+	struct ethernet_config config;
+
+	if (net_eth_get_hw_config(iface, ETHERNET_CONFIG_TYPE_EXTRA_TX_PKT_HEADROOM,
+				  &config) == 0) {
+		reserve += config.extra_tx_pkt_headroom;
+	}
+
+	return net_pkt_alloc_buffer_with_reserve(pkt, size, reserve,
 						 proto, timeout);
 }
 #else