On Tue, Oct 20, 2020 at 02:06:32AM +0200, Andrew Lunn wrote: > I agree with the analysis. This is how i designed it! [...] > So i decided to keep it KISS and not bother with reference counting.
Well, I can't argue with that then... > The other part of the argument is that DSA is stateless, in that there > is no dynamic memory allocation. Drivers are also stateless in terms > of dynamically allocating memory. And is that some sort of design goal? I think you'll run out of things to do very quickly if you set out to never call kmalloc(). > So, what value do this code add? Why do we actually need reference > counting? Well, for one thing, if you have multiple bridge interfaces spanning the ports of a single switch ASIC (and this is not uncommon with port count > 4) then you'll have the timer expiry from one bridge clear the host MDB entries of the other. Weird. And in general, you can't just "add first, delete first" with this type of things. I actually got some pushback from Vivien an year ago on a topic very similar to this: in the other place where DSA is "lazy" and does not implement refcounting, which is VLANs on the CPU port, at least the VLAN is not deleted. That would be more correct, at least, than performing 6 additions, then 1 single deletion which would invalidate the entry for all the other ports. Also, I am taking the opportunity to add the refcounting infrastructure for host FDB entries (this goes back to the patch series about DSA RX filtering). At the moment, host addresses are added from a single type of source (aka, a switchdev HOST_MDB object). But with unicast, there could be more than one sources that a unicast address could be added from: - the MAC addresses of the ports. These addresses should be installed as host FDB entries - the MAC addresses of the upper interfaces. Similar argument here. - the local (master) FDB of the bridge. My plan of record is to offload this using a new switchdev HOST_FDB object, similar to what you've done for HOST_MDB. In this case, having reference counting is pretty much something to have, when an address could come from more than one place, we wouldn't want to break anything. Also, consider what happens when you start having the ability to install the MAC address of the switch ports as a host FDB entry. DSA configures all interfaces to have the same MAC address, which is inherited from the master's MAC address. But you can also change the MAC address of a switch interface. What do you do with the old one? Do you delete it? Do you keep it? If you don't delete it, you might run out of FDB space after enough MAC address changes. If you delete it, you might break traffic for the other switch ports that are still using it...
