linux-kernel - [RFC net-next v3 1/7] queue_api: add support for fetching per queue DMA dev

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Message-ID: <20250815110401.2254214-3-dtatulea@nvidia.com>
Date: Fri, 15 Aug 2025 14:03:42 +0300
From: Dragos Tatulea <dtatulea@...dia.com>
To: <almasrymina@...gle.com>, <asml.silence@...il.com>, "David S. Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman
	<horms@...nel.org>
CC: Dragos Tatulea <dtatulea@...dia.com>, <cratiu@...dia.com>,
	<tariqt@...dia.com>, <parav@...dia.com>, Christoph Hellwig
	<hch@...radead.org>, <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: [RFC net-next v3 1/7] queue_api: add support for fetching per queue DMA dev

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.
- For Multi-PF netdevs [2] queues can be associated to different DMA
devices.

This patch introduces the a queue based interface for allowing drivers
to expose a different DMA device for zerocopy.

[1] Documentation/networking/device_drivers/ethernet/mellanox/mlx5/switchdev.rst
[2] Documentation/networking/multi-pf-netdev.rst

Signed-off-by: Dragos Tatulea <dtatulea@...dia.com>
---
 include/net/netdev_queues.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/net/netdev_queues.h b/include/net/netdev_queues.h
index 6e835972abd1..d4d8c42b809f 100644
--- a/include/net/netdev_queues.h
+++ b/include/net/netdev_queues.h
@@ -127,6 +127,10 @@ void netdev_stat_queue_sum(struct net_device *netdev,
  * @ndo_queue_stop:	Stop the RX queue at the specified index. The stopped
  *			queue's memory is written at the specified address.
  *
+ * @ndo_queue_get_dma_dev: Get dma device for zero-copy operations to be used
+ *			   for this queue. When such device is not available,
+ *			   the function will return NULL.
+ *
  * Note that @ndo_queue_mem_alloc and @ndo_queue_mem_free may be called while
  * the interface is closed. @ndo_queue_start and @ndo_queue_stop will only
  * be called for an interface which is open.
@@ -144,6 +148,8 @@ struct netdev_queue_mgmt_ops {
 	int			(*ndo_queue_stop)(struct net_device *dev,
 						  void *per_queue_mem,
 						  int idx);
+	struct device *		(*ndo_queue_get_dma_dev)(struct net_device *dev,
+							 int idx);
 };
 
 /**
@@ -321,4 +327,18 @@ static inline void netif_subqueue_sent(const struct net_device *dev,
 					 get_desc, start_thrs);		\
 	})
 
+static inline struct device *
+netdev_queue_get_dma_dev(struct net_device *dev, int idx)
+{
+	const struct netdev_queue_mgmt_ops *queue_ops = dev->queue_mgmt_ops;
+	struct device *dma_dev;
+
+	if (queue_ops && queue_ops->ndo_queue_get_dma_dev)
+		dma_dev = queue_ops->ndo_queue_get_dma_dev(dev, idx);
+	else
+		dma_dev = dev->dev.parent;
+
+	return dma_dev && dma_dev->dma_mask ? dma_dev : NULL;
+}
+
 #endif
-- 
2.50.1