On 09/03/2026 15:51, Björn Töpel wrote:

Björn Töpel [off-list ref] writes:

quoted

Maxime Chevallier [off-list ref] writes:

quoted

Hi Björn,

On 08/03/2026 13:40, Björn Töpel wrote:

quoted

Add the kernel-side ETHTOOL_MSG_LOOPBACK_GET,
ETHTOOL_MSG_LOOPBACK_SET, and ETHTOOL_MSG_LOOPBACK_NTF handlers using
the standard ethnl_request_ops infrastructure.

GET collects loopback entries from per-component helpers via
loopback_get_entries(). SET parses the nested entry attributes,
dispatches each to loopback_set_one(), and only sends a notification
when the state is changed.

No components are wired yet.

Signed-off-by: Björn Töpel <bjorn@kernel.org>

At a first glance, what I see is that you're using the GET ->doit method
to retrieve an array of loopback entries. The pefered approach in that
case is to use the GET ->dumpit command instead, issueing a netlink DUMP
request to list all available loopback entities on a given netdev.

If you want some reference on that, take a look at the phy.c + the
'perphy' helpers in net/ethtool/netlink.c

The idea is that you can pass a netdev ifindex in the header of the DUMP
request, which you can use to dump all loopbacks the passed netdev.

You can also check the ethtool code itself, you'll see that when you use
the "ethtool --show-phys eth0" command for example, it issues a DUMP
request to the kernel.

Ah, got it! Thanks!

Thanks for pointing this out! I like how using this can get rid of the
fixed size array in struct ethtool_loopback_cfg, which had a bad smell
to me.

Thinking out loud -- Now, using a similar scheme for loopback requires
some thought around the sub-iterator: Loopback has "multiple"
sub-iterator: (component, [id], name)... or needs scheme to deal with
one sub-iter, and loopback can share a lot of code with the
perphy-functions.

yeah, at some point in the past I played around with the idea of more
generic ethnl helpers for filtered DUMP requests :

https://lore.kernel.org/netdev/20250324104012.367366-1-maxime.chevallier@bootlin.com/ (local)

At that time, the PHY usecase was the main user so I ended-up ditching
that in favor of the more specific perphy helpers we have today, after
discussing the matter with Jakub.

Now that we're seeing more and more ethnl DUMP commands that can take a
netdev ifindex in the request, maybe some of that code above can be useful
for you and we can resurect this approach ?

Maxime 

Björn