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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Saeed Mahameed , tariqt@nvidia.com, cratiu@nvidia.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, Jul 8, 2025 at 4:05=E2=80=AFAM Pavel Begunkov wrote: > > 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 > > --- > > 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_devic= e *dev) > > +{ > > + struct device *dma_dev =3D 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 we= ll. > > 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 netde= v > locking came after devmem got merged, and there are other virt devices th= at > might already be converted. > We never attempt devmem binding on any devices that don't support the queue API, even before the per netdev locking was merged (there was an explicit ops check). even then, dev->dev.parent =3D=3D NULL isn't disasterous, as far as I could surmise from a quick look. Seems to be only used with dma_buf_attach which NULL checks it. --=20 Thanks, Mina