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

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Message-ID: <aPX8di8QX96JvIZY@krikkit>
Date: Mon, 20 Oct 2025 11:10:14 +0200
From: Sabrina Dubroca <sd@...asysnail.net>
To: Hangbin Liu <liuhangbin@...il.com>
Cc: netdev@...r.kernel.org, Jay Vosburgh <jv@...sburgh.net>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jiri Pirko <jiri@...nulli.us>, Simon Horman <horms@...nel.org>,
	Ido Schimmel <idosch@...dia.com>, Shuah Khan <shuah@...nel.org>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Stanislav Fomichev <stfomichev@...il.com>,
	Kuniyuki Iwashima <kuniyu@...gle.com>,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	bridge@...ts.linux.dev
Subject: Re: [PATCHv6 net-next 1/4] net: add a common function to compute
 features for upper devices

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@...hat.com>
> Signed-off-by: Hangbin Liu <liuhangbin@...il.com>

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

Reviewed-by: Sabrina Dubroca <sd@...asysnail.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