net: enable driver support for netmem TX

Drivers need to make sure not to pass netmem dma-addrs to the
dma-mapping API in order to support netmem TX.

Add helpers and netmem_dma_*() helpers that enables special handling of
netmem dma-addrs that drivers can use.

Document in netmem.rst what drivers need to do to support netmem TX.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250508004830.4100853-7-almasrymina@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/net_cachelines/net_device.rst

@@ -10,6 +10,7 @@ Type                                Name                        fastpath_tx_acce
 =================================== =========================== =================== =================== ===================================================================================
 unsigned_long:32                    priv_flags                  read_mostly                             __dev_queue_xmit(tx)
 unsigned_long:1                     lltx                        read_mostly                             HARD_TX_LOCK,HARD_TX_TRYLOCK,HARD_TX_UNLOCK(tx)
+unsigned long:1                     netmem_tx:1;                read_mostly
 char                                name[16]
 struct netdev_name_node*            name_node
 struct dev_ifalias*                 ifalias

Documentation/networking/netdev-features.rst

@@ -188,3 +188,8 @@ Redundancy) frames from one port to another in hardware.
 This should be set for devices which duplicate outgoing HSR (High-availability
 Seamless Redundancy) or PRP (Parallel Redundancy Protocol) tags automatically
 frames in hardware.
+
+* netmem-tx
+
+This should be set for devices which support netmem TX. See
+Documentation/networking/netmem.rst

Documentation/networking/netmem.rst

@@ -19,8 +19,8 @@ Benefits of Netmem :
 * Simplified Development: Drivers interact with a consistent API,
   regardless of the underlying memory implementation.
 
-Driver Requirements
+Driver RX Requirements
-===================
+======================
 
 1. The driver must support page_pool.
 
@@ -77,3 +77,22 @@ Driver Requirements
    that purpose, but be mindful that some netmem types might have longer
    circulation times, such as when userspace holds a reference in zerocopy
    scenarios.
+
+Driver TX Requirements
+======================
+
+1. The Driver must not pass the netmem dma_addr to any of the dma-mapping APIs
+   directly. This is because netmem dma_addrs may come from a source like
+   dma-buf that is not compatible with the dma-mapping APIs.
+
+   Helpers like netmem_dma_unmap_page_attrs() & netmem_dma_unmap_addr_set()
+   should be used in lieu of dma_unmap_page[_attrs](), dma_unmap_addr_set().
+   The netmem variants will handle netmem dma_addrs correctly regardless of the
+   source, delegating to the dma-mapping APIs when appropriate.
+
+   Not all dma-mapping APIs have netmem equivalents at the moment. If your
+   driver relies on a missing netmem API, feel free to add and propose to
+   netdev@, or reach out to the maintainers and/or almasrymina@google.com for
+   help adding the netmem API.
+
+2. Driver should declare support by setting `netdev->netmem_tx = true`

include/linux/netdevice.h

@@ -1772,6 +1772,7 @@ enum netdev_reg_state {
  *	@lltx:		device supports lockless Tx. Deprecated for real HW
  *			drivers. Mainly used by logical interfaces, such as
  *			bonding and tunnels
+ *	@netmem_tx:	device support netmem_tx.
  *
  *	@name:	This is the first field of the "visible" part of this structure
  *		(i.e. as seen by users in the "Space.c" file).  It is the name
@@ -2087,6 +2088,7 @@ struct net_device {
 	struct_group(priv_flags_fast,
 		unsigned long		priv_flags:32;
 		unsigned long		lltx:1;
+		unsigned long		netmem_tx:1;
 	);
 	const struct net_device_ops *netdev_ops;
 	const struct header_ops *header_ops;

include/net/netmem.h

@@ -8,6 +8,7 @@
 #ifndef _NET_NETMEM_H
 #define _NET_NETMEM_H
 
+#include <linux/dma-mapping.h>
 #include <linux/mm.h>
 #include <net/net_debug.h>
 
@@ -276,4 +277,23 @@ static inline unsigned long netmem_get_dma_addr(netmem_ref netmem)
 void get_netmem(netmem_ref netmem);
 void put_netmem(netmem_ref netmem);
 
+#define netmem_dma_unmap_addr_set(NETMEM, PTR, ADDR_NAME, VAL)   \
+	do {                                                     \
+		if (!netmem_is_net_iov(NETMEM))                  \
+			dma_unmap_addr_set(PTR, ADDR_NAME, VAL); \
+		else                                             \
+			dma_unmap_addr_set(PTR, ADDR_NAME, 0);   \
+	} while (0)
+
+static inline void netmem_dma_unmap_page_attrs(struct device *dev,
+					       dma_addr_t addr, size_t size,
+					       enum dma_data_direction dir,
+					       unsigned long attrs)
+{
+	if (!addr)
+		return;
+
+	dma_unmap_page_attrs(dev, addr, size, dir, attrs);
+}
+
 #endif /* _NET_NETMEM_H */