Re: [PATCH net-next v4 7/7] net: devmem: allow binding on rx queues with same DMA devices

[Dragos Tatulea]

On Wed, Aug 20, 2025 at 03:57:32PM -0700, Mina Almasry wrote:
> On Wed, Aug 20, 2025 at 10:14 AM Dragos Tatulea <dtatulea@xxxxxxxxxx> wrote:
> >
> > Multi-PF netdevs have queues belonging to different PFs which also means
> > different DMA devices. This means that the binding on the DMA buffer can
> > be done to the incorrect device.
> >
> > This change allows devmem binding to multiple queues only when the
> > queues have the same DMA device. Otherwise an error is returned.
> >
> > Signed-off-by: Dragos Tatulea <dtatulea@xxxxxxxxxx>
> > ---
> >  net/core/netdev-genl.c | 34 +++++++++++++++++++++++++++++++++-
> >  1 file changed, 33 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/core/netdev-genl.c b/net/core/netdev-genl.c
> > index 0df9c159e515..a8c27f636453 100644
> > --- a/net/core/netdev-genl.c
> > +++ b/net/core/netdev-genl.c
> > @@ -906,6 +906,33 @@ static int netdev_nl_read_rxq_bitmap(struct genl_info *info,
> >         return 0;
> >  }
> >
> > +static struct device *netdev_nl_get_dma_dev(struct net_device *netdev,
> > +                                           unsigned long *rxq_bitmap,
> > +                                           struct netlink_ext_ack *extack)
> > +{
> > +       struct device *dma_dev = NULL;
> > +       u32 rxq_idx, prev_rxq_idx;
> > +
> > +       for_each_set_bit(rxq_idx, rxq_bitmap, netdev->real_num_rx_queues) {
> > +               struct device *rxq_dma_dev;
> > +
> > +               rxq_dma_dev = netdev_queue_get_dma_dev(netdev, rxq_idx);
> > +               /* Multi-PF netdev queues can belong to different DMA devoces.
> > +                * Block this case.
> > +                */
> > +               if (dma_dev && rxq_dma_dev != dma_dev) {
> > +                       NL_SET_ERR_MSG_FMT(extack, "Queue %u has a different dma device than queue %u",
> > +                                          rxq_idx, prev_rxq_idx);
> > +                       return ERR_PTR(-EOPNOTSUPP);
> > +               }
> > +
> > +               dma_dev = rxq_dma_dev;
> > +               prev_rxq_idx = rxq_idx;
> > +       }
> > +
> > +       return dma_dev;
> > +}
> > +
> >  int netdev_nl_bind_rx_doit(struct sk_buff *skb, struct genl_info *info)
> >  {
> >         struct net_devmem_dmabuf_binding *binding;
> > @@ -969,7 +996,12 @@ int netdev_nl_bind_rx_doit(struct sk_buff *skb, struct genl_info *info)
> >         if (err)
> >                 goto err_rxq_bitmap;
> >
> > -       dma_dev = netdev_queue_get_dma_dev(netdev, 0);
> > +       dma_dev = netdev_nl_get_dma_dev(netdev, rxq_bitmap, info->extack);
> > +       if (IS_ERR(dma_dev)) {
> 
> Does this need to be IS_ERR_OR_NULL? AFAICT if all the ndos return
> NULL, then dma_dev will also be NULL, and NULL is not a valid value to
> pass to bind_dmabuf below.
>
netdev_nl_get_dma_dev() signals DMA device mismatch though EOPNOTSUPP.
That's why it is IS_ERR_OR_NULL. I find that doing this at the calling
site would be possible but less accurate.

Or did you mean something else?

Thannks,
Dragos