netdev - Re: [RFC net-next v3 4/7] net/mlx5e: add op for getting netdev DMA device

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Message-ID: <jvbtvbmgqspgfc7q2bprtdtigrhdsrjqf3un2wvxnbydngyc7r@y2sgbxgqkdyi>
Date: Mon, 18 Aug 2025 17:40:09 +0000
From: Dragos Tatulea <dtatulea@...dia.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: asml.silence@...il.com, Saeed Mahameed <saeedm@...dia.com>, 
	Tariq Toukan <tariqt@...dia.com>, Mark Bloch <mbloch@...dia.com>, 
	Leon Romanovsky <leon@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, cratiu@...dia.com, 
	parav@...dia.com, Christoph Hellwig <hch@...radead.org>, netdev@...r.kernel.org, 
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC net-next v3 4/7] net/mlx5e: add op for getting netdev DMA
 device

On Fri, Aug 15, 2025 at 10:37:15AM -0700, Mina Almasry wrote:
> On Fri, Aug 15, 2025 at 4:07 AM Dragos Tatulea <dtatulea@...dia.com> wrote:
> >
> > For zero-copy (devmem, io_uring), the netdev DMA device used
> > is the parent device of the net device. However that is not
> > always accurate for mlx5 devices:
> > - SFs: The parent device is an auxdev.
> > - Multi-PF netdevs: The DMA device should be determined by
> >   the queue.
> >
> > This change implements the DMA device queue API that returns the DMA
> > device appropriately for all cases.
> >
> > Signed-off-by: Dragos Tatulea <dtatulea@...dia.com>
> > ---
> >  .../net/ethernet/mellanox/mlx5/core/en_main.c | 24 +++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> >
> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> > index 21bb88c5d3dc..0e48065a46eb 100644
> > --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> > @@ -5625,12 +5625,36 @@ static int mlx5e_queue_start(struct net_device *dev, void *newq,
> >         return 0;
> >  }
> >
> > +static struct device *mlx5e_queue_get_dma_dev(struct net_device *dev,
> > +                                             int queue_index)
> > +{
> > +       struct mlx5e_priv *priv = netdev_priv(dev);
> > +       struct mlx5e_channels *channels;
> > +       struct device *pdev = NULL;
> > +       struct mlx5e_channel *ch;
> > +
> > +       channels = &priv->channels;
> > +
> > +       mutex_lock(&priv->state_lock);
> > +
> > +       if (queue_index >= channels->num)
> > +               goto out;
> > +
> > +       ch = channels->c[queue_index];
> > +       pdev = ch->pdev;
> 
> This code assumes priv is initialized, and probably that the device is
> up/running/registered. At first I thought that was fine, but now that
> I look at the code more closely, netdev_nl_bind_rx_doit checks if the
> device is present but doesn't seem to check that the device is
> registered.
> 
> I wonder if we should have a generic check in netdev_nl_bind_rx_doit
> for NETDEV_REGISTERED, and if not, does this code handle unregistered
> netdev correctly (like netdev_priv and priv->channels are valid even
> for unregistered mlx5 devices)?
>
netdev_get_by_index_lock() returns non-NULL only when the device is in
state NETDEV_REGISTERED or NETREG_UNINITIALIZED. So I think  that this
check should suffice.

Thanks,
Dragos