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

[Mark Bloch]

On 07/05/2026 14:03, Jiri Pirko wrote:
> Wed, May 06, 2026 at 07:35:10PM +0200, mbloch@xxxxxxxxxx wrote:
>>
>>
>> On 06/05/2026 18:22, Jiri Pirko wrote:
>>> Wed, May 06, 2026 at 02:37:35PM +0200, mbloch@xxxxxxxxxx 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?

I don't think switchdev by default should mean CX4+ in general. If we get
there, I would expect it to be limited to the DPU/BlueField/ECPF case, where
the host PF probe path can depend on the ECPF reaching switchdev. Changing the
default for regular host NIC deployments feels like a much larger compatibility
change.

For the ASIC/NV bit: maybe technically possible, but it feels like the wrong
layer. This is boot/deployment policy, not a persistent hardware property, and
storing it in NV memory would make the state persist across kernels/hosts in a
surprising way.

I do agree the RFC probably went too far by making a generic devlink cmdline
configuration language. Maybe the smaller thing to discuss is only:

devlink=[pci/...]:esw:mode:{legacy|switchdev|switchdev_inactive}

No runtime params, no ordering between different operations, just early eswitch
mode for explicitly selected handles.

@Jakub, I know you wanted something more generic/extensible, but maybe the
generic case belongs in the devlinkd/systemd direction Jiri pointed at, while
the kernel cmdline handles only this early boot eswitch mode case.

Mark