[PATCH v2 net-next] net: Allow SF devices to be used for ZC DMA

[Dragos Tatulea <dtatulea@nvidia.com>]

For zerocopy (io_uring, devmem), there is an assumption that the
parent device can do DMA. However that is not always the case:
scalable function netdevs [1] have the DMA device in the grandparent.

This patch adds a helper for getting the DMA device for a netdev from
its parent or grandparent if necessary. The NULL case is handled in the
callers.

devmem and io_uring are updated accordingly to use this helper instead
of directly using the parent.

[1] Documentation/networking/device_drivers/ethernet/mellanox/mlx5/switchdev.rst

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Cosmin Ratio <cratiu@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
----
Changes in v2 [2]:
- Dropped the Fixes tag.
- Added more documentation as requeseted.
- Renamed the patch title to better reflect its purpose.

Changes in v1 [1]:
- Upgraded from RFC status.
- Dropped driver specific bits for generic solution.
- Implemented single patch as a fix as requested in RFC.
- Handling of multi-PF netdevs will be handled in a subsequent patch
  series.

[1] RFC: https://lore.kernel.org/all/20250702172433.1738947-2-dtatulea@nvidia.com/
[2] v2: https://lore.kernel.org/all/20250709124059.516095-2-dtatulea@nvidia.com/
---
 include/linux/netdevice.h | 21 +++++++++++++++++++++
 io_uring/zcrx.c           |  2 +-
 net/core/devmem.c         | 10 +++++++++-
 3 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 5847c20994d3..53aa63d6e5a3 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -5560,4 +5560,25 @@ extern struct net_device *blackhole_netdev;
 		atomic_long_add((VAL), &(DEV)->stats.__##FIELD)
 #define DEV_STATS_READ(DEV, FIELD) atomic_long_read(&(DEV)->stats.__##FIELD)
 
+static inline struct device *netdev_get_dma_dev(const struct net_device *dev)
+{
+	struct device *dma_dev = dev->dev.parent;
+
+	if (!dma_dev)
+		return NULL;
+
+	/* Common case: dma device is parent device of netdev. */
+	if (dma_dev->dma_mask)
+		return dma_dev;
+
+	/* SF netdevs have an auxdev device as parent, the dma device being the
+	 * grandparent.
+	 */
+	dma_dev = dma_dev->parent;
+	if (dma_dev && dma_dev->dma_mask)
+		return dma_dev;
+
+	return NULL;
+}
+
 #endif	/* _LINUX_NETDEVICE_H */
diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
index 797247a34cb7..93462e5b2207 100644
--- a/io_uring/zcrx.c
+++ b/io_uring/zcrx.c
@@ -584,7 +584,7 @@ int io_register_zcrx_ifq(struct io_ring_ctx *ctx,
 		goto err;
 	}
 
-	ifq->dev = ifq->netdev->dev.parent;
+	ifq->dev = netdev_get_dma_dev(ifq->netdev);
 	if (!ifq->dev) {
 		ret = -EOPNOTSUPP;
 		goto err;
diff --git a/net/core/devmem.c b/net/core/devmem.c
index b3a62ca0df65..881044e0ae0e 100644
--- a/net/core/devmem.c
+++ b/net/core/devmem.c
@@ -183,6 +183,7 @@ net_devmem_bind_dmabuf(struct net_device *dev,
 {
 	struct net_devmem_dmabuf_binding *binding;
 	static u32 id_alloc_next;
+	struct device *dma_dev;
 	struct scatterlist *sg;
 	struct dma_buf *dmabuf;
 	unsigned int sg_idx, i;
@@ -193,6 +194,13 @@ net_devmem_bind_dmabuf(struct net_device *dev,
 	if (IS_ERR(dmabuf))
 		return ERR_CAST(dmabuf);
 
+	dma_dev = netdev_get_dma_dev(dev);
+	if (!dma_dev) {
+		err = -EOPNOTSUPP;
+		NL_SET_ERR_MSG(extack, "Device doesn't support dma");
+		goto err_put_dmabuf;
+	}
+
 	binding = kzalloc_node(sizeof(*binding), GFP_KERNEL,
 			       dev_to_node(&dev->dev));
 	if (!binding) {
@@ -209,7 +217,7 @@ net_devmem_bind_dmabuf(struct net_device *dev,
 
 	binding->dmabuf = dmabuf;
 
-	binding->attachment = dma_buf_attach(binding->dmabuf, dev->dev.parent);
+	binding->attachment = dma_buf_attach(binding->dmabuf, dma_dev);
 	if (IS_ERR(binding->attachment)) {
 		err = PTR_ERR(binding->attachment);
 		NL_SET_ERR_MSG(extack, "Failed to bind dmabuf to device");
-- 
2.43.0