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

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Message-ID: <CAHS8izO327v1ZXnpqiyBRyO1ntgycVBG9ZLGMdCv4tg_5wBWng@mail.gmail.com>
Date: Fri, 15 Aug 2025 10:37:15 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Dragos Tatulea <dtatulea@...dia.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 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)?


-- 
Thanks,
Mina