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Miller" , Jonathan Corbet , Shuah Khan , Simon Horman , Saeed Mahameed , Leon Romanovsky , Tariq Toukan , Andrew Morton , "Borislav Petkov (AMD)" , Randy Dunlap , Dave Hansen , Christian Brauner , Petr Mladek , "Peter Zijlstra (Intel)" , Thomas Gleixner , Pawan Gupta , Dapeng Mi , Kees Cook , Marco Elver , Eric Biggers , Li RongQing , "Paul E. McKenney" , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, linux-rdma@vger.kernel.org Subject: Re: [RFC net-next 0/4] devlink: Add boot-time defaults Message-ID: References: <20260506123739.1959770-1-mbloch@nvidia.com> <3f9215c4-7c84-46d9-ba74-30dabe24db09@nvidia.com> Precedence: bulk X-Mailing-List: linux-rdma@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3f9215c4-7c84-46d9-ba74-30dabe24db09@nvidia.com> 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?