linux-kernel - Re: [PATCH net-next 10/10] net/mlx5e: Use the 'num

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Message-ID: <aMGkaDoZpmOWUA_L@mini-arch>
Date: Wed, 10 Sep 2025 09:16:40 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Tariq Toukan <tariqt@...dia.com>
Cc: Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Jiri Pirko <jiri@...nulli.us>, Jonathan Corbet <corbet@....net>,
	Leon Romanovsky <leon@...nel.org>, Jason Gunthorpe <jgg@...pe.ca>,
	Saeed Mahameed <saeedm@...dia.com>, Mark Bloch <mbloch@...dia.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-rdma@...r.kernel.org, bpf@...r.kernel.org,
	Gal Pressman <gal@...dia.com>, Cosmin Ratiu <cratiu@...dia.com>,
	Dragos Tatulea <dtatulea@...dia.com>, Jiri Pirko <jiri@...dia.com>
Subject: Re: [PATCH net-next 10/10] net/mlx5e: Use the 'num_doorbells'
 devlink param

On 09/10, Tariq Toukan wrote:
> From: Cosmin Ratiu <cratiu@...dia.com>
> 
> Use the new devlink param to control how many doorbells mlx5e devices
> allocate and use. The maximum number of doorbells configurable is capped
> to the maximum number of channels. This only applies to the Ethernet
> part, the RDMA devices using mlx5 manage their own doorbells.
> 
> Signed-off-by: Cosmin Ratiu <cratiu@...dia.com>
> Reviewed-by: Dragos Tatulea <dtatulea@...dia.com>
> Signed-off-by: Tariq Toukan <tariqt@...dia.com>
> ---
>  Documentation/networking/devlink/mlx5.rst     |  8 ++++++
>  .../net/ethernet/mellanox/mlx5/core/devlink.c | 26 +++++++++++++++++++
>  .../ethernet/mellanox/mlx5/core/en_common.c   | 15 ++++++++++-
>  3 files changed, 48 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/networking/devlink/mlx5.rst b/Documentation/networking/devlink/mlx5.rst
> index 60cc9fedf1ef..0650462b3eae 100644
> --- a/Documentation/networking/devlink/mlx5.rst
> +++ b/Documentation/networking/devlink/mlx5.rst
> @@ -45,6 +45,14 @@ Parameters
>       - The range is between 1 and a device-specific max.
>       - Applies to each physical function (PF) independently, if the device
>         supports it. Otherwise, it applies symmetrically to all PFs.
> +   * - ``num_doorbells``
> +     - driverinit
> +     - This controls the number of channel doorbells used by the netdev. In all
> +       cases, an additional doorbell is allocated and used for non-channel
> +       communication (e.g. for PTP, HWS, etc.). Supported values are:
> +       - 0: No channel-specific doorbells, use the global one for everything.
> +       - [1, max_num_channels]: Spread netdev channels equally across these
> +         doorbells.

Do you have any guidance on this number? Why would the user want
`num_doorbells < num_doorbells` vs `num_doorbells == num_channels`?

IOW, why not allocate the same number of doorbells as the number of
channels and do it unconditionally without devlink param? Are extra
doorbells causing any overhead in the non-contended case?