Re: [PATCH net-next] ethtool: Fix set RXFH for drivers without RXFH fields support

[Jakub Kicinski <kuba@kernel.org>]

On Wed, 9 Jul 2025 18:32:51 +0300 Gal Pressman wrote:
> Some drivers (e.g., mlx4_en) support ->set_rxfh() functionality (such as
> setting hash function), but not setting the RXFH fields.
> 
> The requirement of ->get_rxfh_fields() in ethtool_set_rxfh() is there to
> verify that we have no conflict with the RSS fields options, if it
> doesn't exist then there is no point in doing the check.
> Soften the check in ethtool_set_rxfh() so it doesn't fail when
> ->get_rxfh_fields() doesn't exist.  
> 
> This fixes the following error:
> $ ethtool --set-rxfh-indir eth2 hfunc xor
> Cannot set RX flow hash configuration: Operation not supported

Ah, thanks for the fix!

In this case I wonder if we wouldn't be better off returning early 
in ethtool_check_flow_types() if input_xfrm is 0 or NO_CHANGE.
Most drivers will have get_rxfh_fields - still there's no point
in doing the check if they have empty ops->supported_input_xfrm

We could add a:

	if (WARN_ON(ops->supported_input_xfrm && !ops->get_rxfh_fields))
		return -EINVAL;

into ethtool_check_ops() and we'd be both safe and slightly faster.

WDYT?