Re: MRP netlink interface

[Nikolay Aleksandrov <nikolay@cumulusnetworks.com>]

On 25/05/2020 14:28, Horatiu Vultur wrote:
> Hi,
> 
> While I was working on adding support for MRA role to MRP, I noticed that I
> might have some issues with the netlink interface, so it would be great if you
> can give me an advice on how to continue.
> 
> First a node with MRA role can behave as a MRM(Manager) or as a
> MRC(Client). The behaviour is decided by the priority of each node. So
> to have this functionality I have to extend the MRP netlink interface
> and this brings me to my issues.
> 
> My first approach was to extend the 'struct br_mrp_instance' with a field that
> contains the priority of the node. But this breaks the backwards compatibility,
> and then every time when I need to change something, I will break the backwards
> compatibility. Is this a way to go forward?
> 
> Another approach is to restructure MRP netlink interface. What I was thinking to
> keep the current attributes (IFLA_BRIDGE_MRP_INSTANCE,
> IFLA_BRIDGE_MRP_PORT_STATE,...) but they will be nested attributes and each of
> this attribute to contain the fields of the structures they represents.
> For example:
> [IFLA_AF_SPEC] = {
>     [IFLA_BRIDGE_FLAGS]
>     [IFLA_BRIDGE_MRP]
>         [IFLA_BRIDGE_MRP_INSTANCE]
>             [IFLA_BRIDGE_MRP_INSTANCE_RING_ID]
>             [IFLA_BRIDGE_MRP_INSTANCE_P_IFINDEX]
>             [IFLA_BRIDGE_MRP_INSTANCE_S_IFINDEX]
>         [IFLA_BRIDGE_MRP_RING_ROLE]
>             [IFLA_BRIDGE_MRP_RING_ROLE_RING_ID]
>             [IFLA_BRIDGE_MRP_RING_ROLE_ROLE]
>         ...
> }
> And then I can parse each field separately and then fill up the structure
> (br_mrp_instance, br_mrp_port_role, ...) which will be used forward.
> Then when this needs to be extended with the priority it would have the
> following format:
> [IFLA_AF_SPEC] = {
>     [IFLA_BRIDGE_FLAGS]
>     [IFLA_BRIDGE_MRP]
>         [IFLA_BRIDGE_MRP_INSTANCE]
>             [IFLA_BRIDGE_MRP_INSTANCE_RING_ID]
>             [IFLA_BRIDGE_MRP_INSTANCE_P_IFINDEX]
>             [IFLA_BRIDGE_MRP_INSTANCE_S_IFINDEX]
>             [IFLA_BRIDGE_MRP_INSTANCE_PRIO]
>         [IFLA_BRIDGE_MRP_RING_ROLE]
>             [IFLA_BRIDGE_MRP_RING_ROLE_RING_ID]
>             [IFLA_BRIDGE_MRP_RING_ROLE_ROLE]
>         ...
> }
> And also the br_mrp_instance will have a field called prio.
> So now, if the userspace is not updated to have support for setting the prio
> then the kernel will use a default value. Then if the userspace contains a field
> that the kernel doesn't know about, then it would just ignore it.
> So in this way every time when the netlink interface will be extended it would
> be backwards compatible.
> 
> If it is not possible to break the compatibility then the safest way is to
> just add more attributes under IFLA_BRIDGE_MRP but this would just complicate
> the kernel and the userspace and it would make it much harder to be extended in
> the future.
> 
> My personal choice would be the second approach, even if it breaks the backwards
> compatibility. Because it is the easier to go forward and there are only 3
> people who cloned the userspace application
> (https://github.com/microchip-ung/mrp/graphs/traffic). And two of
> these unique cloners is me and Allan.
> 
> So if you have any advice on how to go forward it would be great.
> 

IIRC this is still in net-next only, right? If so - now would be the time to change it.
Once it goes into a release, we'll be stuck with workarounds. So I'd go for solution 2).

I haven't cloned it, but I do sync your user-space mrp repo to check against the patches. :)