Re: [PATCH net-next v4 2/6] netdev_features: remove unused __UNUSED_NETIF_F_1

On Thu, Aug 22, 2024 at 5:24 PM Alexander Lobakin
[off-list ref] wrote:

From: Eric Dumazet <edumazet@google.com>
Date: Wed, 21 Aug 2024 17:43:16 +0200

quoted

On Wed, Aug 21, 2024 at 5:07 PM Alexander Lobakin
[off-list ref] wrote:

quoted

NETIF_F_NO_CSUM was removed in 3.2-rc2 by commit 34324dc2bf27
("net: remove NETIF_F_NO_CSUM feature bit") and became
__UNUSED_NETIF_F_1. It's not used anywhere in the code.
Remove this bit waste.

It wasn't needed to rename the flag instead of removing it as
netdev features are not uAPI/ABI. Ethtool passes their names
and values separately with no fixed positions and the userspace
Ethtool code doesn't have any hardcoded feature names/bits, so
that new Ethtool will work on older kernels and vice versa.

This is only true for recent enough ethtool (>= 3.4)

You might refine the changelog to not claim this "was not needed".

Back in 2011 (and linux-2.6.39) , this was needed for sure.

I am not sure we have a documented requirement about ethtool versions.

But how then Ethtool < 3.4 works with the latest kernels? I believe we
already moved some bits and/or removed some features or it's not true?

Presumably most of the 'old and useful' bits are at the same location,
or ethtool has been updated by distros.

I could try building it, not sure it would build though. How do you
think then we should approach this? Maybe document the requirement?
I don't think we should leave the features as they are and sit with no
bits available only to support ancient Ethtool versions.

I was simply suggesting to correct the changelog, and make clear we
need a recent enough ethtool.

We can not simply say that ethtool always supported the modern way
(ETH_SS_FEATURES)