Re: RFC: dsa slave open handling

On Mon, Mar 07, 2011 at 08:35:47PM +0100, Peter Korsgaard wrote:

 >> Currently the state of the dsa master device and slave devices are not
 >> synchronized which leads to various issues. With dsa, the master device
 >> needs to be running before you can use the slave devices, but it
 >> shouldn't be used directly for I/O.

 Lennert> I think this should not be handled any differently than VLAN
 Lennert> interfaces, where eth0 needs to be up before you can exchange
 Lennert> packets over eth0.123.

 Lennert> IOW, if you want to do DHCP over wan (a DSA-tunneled interface
 Lennert> over eth0), then just use an initramfs that sets eth0 up and
 Lennert> then runs a DHCP client on wan.

True. There are a few differences between vlans and dsa devices though,
which might make us want to handle them differently:

- VLANs are configured at runtime from userspace, DSA devices are
  configured directly in kernel code (E.G platform devices) - So by
  definition you cannot use the autconfig / nfsroot stuff on VLANs as
  they are not created before userspace runs, but DSA devices are
  basically indistinguishable from "real" devices from the POV of
  userspace.

Sure.  That doesn't change the argument, though.  In fact, in most
situations, you'll e.g. want to bridge the lan[1-4] interfaces
together, and if you then want to DHCP over that bridge, you cannot
explain that to the kernel autoconfigurator either.

- VLANs relation to their upstream ethernet device is clearly visible
  (E.G through brctl), whereas the details about what interface a DSA
  switch is connected to isn't.

commit c084080151e1de92159f8437fde34b6e5bebe35e
Author: Lennert Buytenhek [off-list ref]
Date:   Fri Mar 20 09:49:49 2009 +0000

    dsa: set ->iflink on slave interfaces to the ifindex of the parent
    
    ..so that we can parse the DSA topology from 'ip link' output:
    
    1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue
    2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
    3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
    4: lan1@eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue
    5: lan2@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue
    6: lan3@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue
    7: lan4@eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue
    
    Signed-off-by: Lennert Buytenhek [off-list ref]
    Signed-off-by: David S. Miller [off-list ref]


cheers,
Lennert