Re: [PATCHv6 net-next 1/4] net: add a common function to compute features for upper devices

Sabrina Dubroca
2025-10-20T09:10:20Z

2025-10-17, 03:41:52 +0000, Hangbin Liu wrote:
> Some high level software drivers need to compute features from lower
> devices. But each has their own implementations and may lost some
> feature compute. Let's use one common function to compute features
> for kinds of these devices.
>
> The new helper uses the current bond implementation as the reference
> one, as the latter already handles all the relevant aspects: netdev
> features, TSO limits and dst retention.
>
> Suggested-by: Paolo Abeni <pabeni@redhat.com>
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>

No objection to this patch/series, just a nit and some discussion below, so:

Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>

[...]
> +/**
> + * netdev_compute_master_upper_features - compute feature from lowers

nit: I'm slightly annoyed (that's not quite the right word, sorry)
that we're adding a new function to "compute features" that doesn't
touch netdev->features, but I can't come up with a better name
(the best I got was "compute extra features" and it doesn't help).

> + * @dev: the upper device
> + * @update_header: whether to update upper device's header_len/headroom/tailroom
> + *
> + * Recompute the upper device's feature based on all lower devices.
> + */
> +void netdev_compute_master_upper_features(struct net_device *dev, bool update_header)
> +{
[...]
> + netif_set_tso_max_segs(dev, tso_max_segs);
> + netif_set_tso_max_size(dev, tso_max_size);
> +
> + netdev_change_features(dev);

Maybe a dumb idea: I'm wondering if we're doing this from the wrong
side.

Right now we have:

[some device op] -> [this new function] -> netdev_change_features -> __netdev_update_features -> ndo_fix_features

Would it make more sense to go instead:

[some device op] -> netdev_change_features -> __netdev_update_features -> ndo_fix_features -> [this new function]

?

Possible benefit: not forgetting to fix up the "extra" features in
some cases? (ie calling netdev_change_features when we should have
called netdev_compute_master_upper_features)

> +}

--
Sabrina