netdev - Re: [RFC PATCH net-next 1/7] net: ethtool: netlink: introduce ethnl_update

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Date:   Wed, 17 Aug 2022 14:05:38 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Rui Sousa <rui.sousa@....com>,
        Ferenc Fejes <ferenc.fejes@...csson.com>
Subject: Re: [RFC PATCH net-next 1/7] net: ethtool: netlink: introduce
 ethnl_update_bool()

On Wed, Aug 17, 2022 at 12:00:37PM +0000, Vladimir Oltean wrote:
> On Wed, Aug 17, 2022 at 02:52:56PM +0300, Vladimir Oltean wrote:
> > On Wed, Aug 17, 2022 at 01:27:29PM +0200, Michal Kubecek wrote:
> > > On Wed, Aug 17, 2022 at 01:29:14AM +0300, Vladimir Oltean wrote:
> > > > For a reason I can't really understand, ethnl_update_bool32() exists,
> > > > but the plain function that operates on a boolean value kept in an
> > > > actual u8 netlink attribute doesn't.
> > > 
> > > I can explain that: at the moment these helpers were introduced, only
> > > members of traditional structures shared with ioctl interface were
> > > updated and all attributes which were booleans logically were
> > > represented as u32 in them so that no other helper was needed back then.
> > 
> > Thanks, but the internal data structures of the kernel did not
> > necessitate boolean netlink attributes to be promoted to u32 just
> > because the ioctl interface did it that way; or did they?
> > 
> > Or otherwise said, is there a technical requirement that if a boolean is
> > passed to the kernel as u32 via ioctl, it should be passed as u32 via
> > netlink too?
> 
> Ah, don't mind me... By the time I wrote this email, I forgot that
> ethnl_update_bool32() also calls nla_get_u8(). All clear now.

Yes, it expects a NLA_U8 attribute. But it does not really matter that
much as even NLA_U8 still occupies 4 bytes in the message because of the
padding. (Which is why I generally prefer using NLA_U32 attributes
unless the value is naturally 8- or 16-bit and we are absolutely sure we
will never need more.)

Michal

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