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

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Message-ID: <CANn89i+qJa8FSwdxkK76NSz2Wi4OxP56edFmJ14Zok8BpYQFjQ@mail.gmail.com>
Date: Thu, 22 Aug 2024 18:12:18 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, David Ahern <dsahern@...nel.org>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Andrew Lunn <andrew@...n.ch>, 
	Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	nex.sw.ncis.osdt.itp.upstreaming@...el.com, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: 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
<aleksander.lobakin@...el.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Wed, 21 Aug 2024 17:43:16 +0200
>
> > On Wed, Aug 21, 2024 at 5:07 PM Alexander Lobakin
> > <aleksander.lobakin@...el.com> wrote:
> >>
> >> 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)