netdev - Re: [RFC PATCH net-next v3] ethtool: Improve compatibility between netlink and ioctl interfaces

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Message-ID: <20201029090328.jwl7w7noeib3d4cz@lion.mk-sys.cz>
Date:   Thu, 29 Oct 2020 10:03:28 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        davem@...emloft.net, f.fainelli@...il.com, andrew@...n.ch,
        David.Laight@...lab.com, mlxsw@...dia.com,
        Ido Schimmel <idosch@...dia.com>
Subject: Re: [RFC PATCH net-next v3] ethtool: Improve compatibility between
 netlink and ioctl interfaces

On Wed, Oct 28, 2020 at 07:34:36PM +0200, Ido Schimmel wrote:
> On Wed, Oct 28, 2020 at 01:53:39AM +0100, Michal Kubecek wrote:
> > On Tue, Oct 27, 2020 at 02:53:05PM -0700, Jakub Kicinski wrote:
> > > 
> > > I did not look at the legacy code but I'm confused by what you wrote.
> > > 
> > > IIUC for ioctl it's the user space that sets the advertised.
> > > For netlink it's the kernel.
> > > So how does the legacy flag make the kernel behave like it used to?
> > 
> > The idea why I suggested "legacy" as the name was that it allowed
> > ethtool to preserve the old behaviour (without having to query for
> > supported modes first). But from this point of view it's indeed a bit
> > confusing.
> 
> I think it would be best to solve this by having user space query the
> kernel for supported link modes if autoneg is being enabled without
> additional parameters. Then user space will issue a set request with
> ETHTOOL_A_LINKMODES_OURS being set to all supported link modes.
> 
> It does not require kernel changes and would be easier on users that
> currently need to resort to old ethtool despite having a kernel that
> supports netlink-based ethtool.

That would certainly be a solution. I'm not exactly happy about having
to issue two requests but (1) it would be limited to specific case with
"autoneg on" without advertise, speed and duplex (and lanes, when/if
it's introduced), (2) we would need an extra request to check support of
the flag anyway and (3) supported modes of a device are unlikely to
change so that we don't have to worry about races.

Michal