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

[Alexander Lobakin <aleksander.lobakin@intel.com>]

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

> On Wed, Aug 21, 2024 at 5:07 PM Alexander Lobakin
> <aleksander.lobakin@intel.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?

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.

Thanks,
Olek