Re: Changing devlink port flavor dynamically for DSA

[Florian Fainelli <f.fainelli@gmail.com>]

On 4/6/2020 11:04 AM, Jiri Pirko wrote:
> Sun, Apr 05, 2020 at 10:42:29PM CEST, f.fainelli@gmail.com wrote:
>> Hi all,
>>
>> On a BCM7278 system, we have two ports of the switch: 5 and 8, that
>> connect to separate Ethernet MACs that the host/CPU can control. In
>> premise they are both interchangeable because the switch supports
>> configuring the management port to be either 5 or 8 and the Ethernet
>> MACs are two identical instances.
>>
>> The Ethernet MACs are scheduled differently across the memory controller
>> (they have different bandwidth and priority allocations) so it is
>> desirable to select an Ethernet MAC capable of sustaining bandwidth and
>> latency for host networking. Our current (in the downstream kernel) use
>> case is to expose port 5 solely as a control end-point to the user and
>> leave it to the user how they wish to use the Ethernet MAC behind port
>> 5. Some customers use it to bridge Wi-Fi traffic, some simply keep it
>> disabled. Port 5 of that switch does not make use of Broadcom tags in
>> that case, since ARL-based forwarding works just fine.
>>
>> The current Device Tree representation that we have for that system
>> makes it possible for either port to be elected as the CPU port from a
>> DSA perspective as they both have an "ethernet" phandle property that
>> points to the appropriate Ethernet MAC node, because of that the DSA
>> framework treats them as CPU ports.
>>
>> My current line of thinking is to permit a port to be configured as
>> either "cpu" or "user" flavor and do that through devlink. This can
>> create some challenges but hopefully this also paves the way for finally
>> supporting "multi-CPU port" configurations. I am thinking something like
>> this would be how I would like it to be configured:
>>
>> # First configure port 8 as the new CPU port
>> devlink port set pci/0000:01:00.0/8 type cpu
>> # Now unmap port 5 from being a CPU port
>> devlink port set pci/0000:01:00.0/1 type eth
> 
> You are mixing "type" and "flavour".
> 
> Flavours: cpu/physical. I guess that is what you wanted to set, correct?

Correct, flavor is really what we want to change here.

> 
> I'm not sure, it would make sense. The CPU port is still CPU port, it is
> just not used. You can never make is really "physical", am I correct? 

True, although with DSA as you may know if we have a DSA_PORT_TYPE_CPU
(or DSA_PORT_TYPE_DSA), then we do not create a corresponding net_device
instance because that would duplicate the Ethernet MAC net_device. This
is largely the reason for suggesting doing this via devlink (so that we
do not rely on a net_device handle). So by changing from a
DSA_PORT_TYPE_CPU flavor to DSA_PORT_TYPE_USER, this means you would now
see a corresponding net_device instance. Conversely when you migrate
from DSA_PORT_TYPE_USER to DSA_PORT_TYPE_CPU, the corresponding
net_device would be removed.

Or maybe we finally bite the bullet and create net_device representors
for all port types...

> 
> 
> btw, we already implement port "type" setting. To "eth" and "ib". This
> is how you can change the type of fabric for mlx4 driver.
> 
> 
>>
>> and this would do a simple "swap" of all user ports being now associated
>> with port 8, and no longer with port 5, thus permitting port 5 from
>> becoming a standard user port. Or maybe, we need to do this as an atomic
>> operation in order to avoid a switch being configured with no CPU port
>> anymore, so something like this instead:
>>
>> devlink port set pci/0000:01:00.0/5 type eth mgmt pci/0000:01:00.0/8
>>
>> The latter could also be used to define groups of ports within a switch
>> that has multiple CPU ports, e.g.:
>>
>> # Ports 1 through 4 "bound" to CPU port 5:
>>
>> for i in $(seq 0 3)
>> do
>> 	devlink port set pci/0000:01:00.0/$i type eth mgmt pci/0000:01:00.0/5
>> done
>>
>> # Ports 7 bound to CPU port 8:
>>
>> devlink port set pci/0000:01:00.0/1 type eth mgmt pci/0000:01:00.0/8
> 
> It is basically a mapping of physical port to CPU port, isn't it?
> 
> How about something like?
> devlink port set pci/0000:01:00.0/1 cpu_master pci/0000:01:00.0/5
> devlink port set pci/0000:01:00.0/2 cpu_master pci/0000:01:00.0/5
> devlink port set pci/0000:01:00.0/7 cpu_master pci/0000:01:00.0/8
> 
> If CPU port would have 0 mapped ports, it would mean it is disabled.
> What do you think?

Yes, this makes sense.
-- 
Florian