Re: [RFC net-next 1/4] net: Allow non parent devices to be used for ZC DMA

[Pavel Begunkov]

On 7/2/25 18:24, Dragos Tatulea wrote:
> 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@xxxxxxxxxx>
> ---
>   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)

dev->dev.parent is NULL for veth and I assume other virtual devices as well.

Mina, can you verify that devmem checks that? Seems like veth is rejected
by netdev_need_ops_lock() in netdev_nl_bind_rx_doit(), but IIRC per netdev
locking came after devmem got merged, and there are other virt devices that
might already be converted.

> +		dma_dev = NULL;
> +
> +	return dma_dev;
> +}
> +
>   #endif	/* _LINUX_NETDEVICE_H */

--
Pavel Begunkov