Re: [PATCH RFC net-next 0/3] Multi-CPU DSA support

[Ansuel Smith]

On Sun, Apr 11, 2021 at 08:01:35PM +0200, Marek Behun wrote:
> On Sat, 10 Apr 2021 15:34:46 +0200
> Ansuel Smith <ansuelsmth@xxxxxxxxx> wrote:
> 
> > Hi,
> > this is a respin of the Marek series in hope that this time we can
> > finally make some progress with dsa supporting multi-cpu port.
> > 
> > This implementation is similar to the Marek series but with some tweaks.
> > This adds support for multiple-cpu port but leave the driver the
> > decision of the type of logic to use about assigning a CPU port to the
> > various port. The driver can also provide no preference and the CPU port
> > is decided using a round-robin way.
> 
> In the last couple of months I have been giving some thought to this
> problem, and came up with one important thing: if there are multiple
> upstream ports, it would make a lot of sense to dynamically reallocate
> them to each user port, based on which user port is actually used, and
> at what speed.
> 
> For example on Turris Omnia we have 2 CPU ports and 5 user ports. All
> ports support at most 1 Gbps. Round-robin would assign:
>   CPU port 0 - Port 0
>   CPU port 1 - Port 1
>   CPU port 0 - Port 2
>   CPU port 1 - Port 3
>   CPU port 0 - Port 4
> 
> Now suppose that the user plugs ethernet cables only into ports 0 and 2,
> with 1, 3 and 4 free:
>   CPU port 0 - Port 0 (plugged)
>   CPU port 1 - Port 1 (free)
>   CPU port 0 - Port 2 (plugged)
>   CPU port 1 - Port 3 (free)
>   CPU port 0 - Port 4 (free)
> 
> We end up in a situation where ports 0 and 2 share 1 Gbps bandwidth to
> CPU, and the second CPU port is not used at all.
> 
> A mechanism for automatic reassignment of CPU ports would be ideal here.
> 
> What do you guys think?
> 
> Marek

A function called on every port change that checks the connected ports and
reassign the CPU based on that. Fact is that most of the time devices
have at least 2 ethernet port connected, one for the wan traffic and
other for some LAN device, so some type of preference from the switch
driver is needed, to also try to skip some problematic switch that have
CPU port with different supported features. A good idea but could be
overkill since we have seen at most devices with max 2 CPU port.