Re: [RFC net-next 0/4] devlink: Add boot-time defaults

[Jiri Pirko <jiri@resnulli.us>]

Wed, May 06, 2026 at 07:35:10PM +0200, mbloch@nvidia.com wrote:
>
>
>On 06/05/2026 18:22, Jiri Pirko wrote:
>> Wed, May 06, 2026 at 02:37:35PM +0200, mbloch@nvidia.com wrote:
>>> This series adds a devlink= kernel command line parameter for applying
>>> selected devlink settings during device initialization.
>>>
>>> Following a discussion with Jakub[1], I am sending this RFC to get the
>>> conversation moving. I started from Jakub's example/request and extended
>>> it to cover requirements from production systems and configurations that
>>> customers use.
>>>
>>> One important caveat is that the parsing logic in this RFC was written
>>> with AI assistance. I am also not sure whether the resulting syntax and
>>> parser are too complex for a kernel command line interface. This is part
>>> of why I am sending it as an RFC: to understand what direction and level
>>> of complexity would be acceptable to people.
>>>
>>> The implementation is intended to support the following properties:
>>>
>>> - A system may have multiple devlink devices that usually need the same
>>>  configuration. For a configuration such as eswitch mode switchdev, a
>>>  user should be able to specify multiple devices to which that
>>>  configuration applies.
>>>
>>> - There may be ordering dependencies between options. For example, in
>>>  mlx5, flow_steering_mode should be set before moving to switchdev.
>>>  With this in mind, defaults are applied per device in the left-to-right
>>>  order in which they appear on the command line.
>>>
>>> The intent is to let deployments set devlink defaults before normal
>>> userspace orchestration runs, while still using devlink concepts and
>> 
>> "defaults before normal userspace orchestrarion". I read it as config
>> before config, which eventually could be skipped.
>> 
>> 
>>> driver callbacks rather than adding driver-specific module parameters.
>>> A default is scoped to one or more devlink handles, for example:
>>>
>>>  devlink=[pci/0000:08:00.0]:esw:mode:switchdev
>>>  devlink=[pci/0000:08:00.0]:param:flow_steering_mode:smfs
>>>  devlink=[pci/0000:08:00.0,pci/0000:08:00.1]:param:flow_steering_mode:hmfs,[pci/0000:08:00.0,pci/0000:08:00.1]:esw:mode:switchdev
>> 
>> I don't like this. What you do, you are basically introducing user
>> configuration tool on kernel cmdline.
>> 
>> The same you would achieve with a proper userspace tool/daemon.
>> I did try to come up with it and push it here:
>> https://github.com/systemd/systemd/pull/37393
>> That didn't get merged for unknown reason, but the idea is sound. You
>> provide configuration files for devlink object and systemd-devlinkd
>> will apply when they appear. Wouldn't this help your case?
>
>I agree that systemd-devlinkd is the right shape for normal
>devlink configuration, and it could probably replace the udev/devlink
>plumbing we use today.
>
>The case I am trying to cover is earlier than that.
>
>On BlueField/ECPF/DPU systems, the host PF driver cannot always finish
>probing independently of the ECPF side. When the ECPF is the eswitch
>manager, the host PF is kept in initializing state until the ECPF eswitch
>side is set up and mlx5 enables the external host PF HCA. That happens as
>part of moving the ECPF to switchdev.
>
>Today userspace observes the ECPF instance and then switches the
>mode through devlink, usually via udev or similar plumbing. That still
>leaves a window where the ECPF has probed, userspace has not applied the
>mode yet, and the host PF is waiting. With many ECPFs this becomes visible
>in host PF probe/boot time. A daemon reacting to the devlink object
>appearing can make the userspace side cleaner, but it still runs after the
>device has appeared and after userspace scheduling/uevent handling.
>
>Long term, for these DPU deployments, we would like mlx5 to initialize
>directly in switchdev. I am hesitant to make that unconditional because it
>changes existing behavior and there is no early opt-out before probe. The
>cmdline parameter was meant as an explicit opt-in middle step: ask the
>driver to apply the same devlink operation during init, before this path
>depends on userspace.
>
>We previously tried to address this with an mlx5 module parameter. By
>design, that was too coarse: it applied to all mlx5 devices handled by the
>module. That makes it usable only for narrow DPU-only configurations. The
>devlink-handle based cmdline syntax was intended to keep the opt-in scoped
>to the specific devices that need this early switchdev transition.

The switchdev mode was introduced at roughly the time CX4 was out. What
stopped us from making it default for CX4+ ?

Introducing this horrible plumbing only bacause we were not able to
change the default sounds so absurd.

Can we write the default mode as a bit in ASIC NV memory perhaps? Simple
devlink cmode permanent param to write it, the driver can read this bit
during init to decide the init flow path?