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

[Christoph Hellwig <hch@infradead.org>]

On Mon, Jul 14, 2025 at 06:11:36PM -0700, Jakub Kicinski wrote:
> > +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;
> > +}
> 
> LGTM, but we need a better place for this function. netdevice.h is
> included directly by 1.5k files, and indirectly by probably another 5k.
> It's not a great place to put random helpers with 2 callers. 
> Maybe net/netdev_rx_queue.h and net/core/netdev_rx_queue.c?
> I don't think it needs to be a static inline either.

The whole concept is also buggy.  Trying to get a dma-able device by
walking down from an upper level construct like the netdevice can't work
reliably.  You'll need to explicitly provide the dma_device using either
a method or a pointer to it instead of this guesswork.