[RFC net-next 1/4] net: Allow non parent 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:
for example mlx5 SF devices have an auxiliary device as a parent.

This patch introduces the possibility for the driver to specify
another DMA device to be used via the new dma_dev field. The field
should be set before register_netdev().

A new helper function is added to get the DMA device or return NULL.
The callers can check for NULL and fail early if the device is
not capable of DMA.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
---
 include/linux/netdevice.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 5847c20994d3..83faa2314c30 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2550,6 +2550,9 @@ struct net_device {
 
 	struct hwtstamp_provider __rcu	*hwprov;
 
+	/* To be set by devices that can do DMA but not via parent. */
+	struct device		*dma_dev;
+
 	u8			priv[] ____cacheline_aligned
 				       __counted_by(priv_len);
 } ____cacheline_aligned;
@@ -5560,4 +5563,14 @@ 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->dma_dev ? dev->dma_dev : dev->dev.parent;
+
+	if (!dma_dev->dma_mask)
+		dma_dev = NULL;
+
+	return dma_dev;
+}
+
 #endif	/* _LINUX_NETDEVICE_H */
-- 
2.50.0