netdev - Re: [PATCH] ethtool: ETHTOOL_SFEATURES: remove NETIF_F

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Message-ID: <20110527152809.GA7620@rere.qmqm.pl>
Date:	Fri, 27 May 2011 17:28:10 +0200
From:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] ethtool: ETHTOOL_SFEATURES: remove NETIF_F_COMPAT
 return

On Fri, May 27, 2011 at 03:13:46PM +0100, Ben Hutchings wrote:
> On Tue, 2011-05-24 at 23:59 +0200, Michał Mirosław wrote:
> > On Tue, May 24, 2011 at 03:39:30PM -0400, David Miller wrote:
> > > From: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> > > Date: Tue, 24 May 2011 11:14:37 +0200
> > > > On Thu, May 19, 2011 at 12:03:31PM +0200, Michał Mirosław wrote:
> > > >> On Mon, May 16, 2011 at 02:09:58PM -0400, David Miller wrote:
> > > >> > You guys really need to sort this out properly.
> > > >> > Please resubmit whatever final solution is agreed upon.
> > > >> I noticed that v2.6.39 was tagged today. We should definitely remove
> > > >> NETIF_F_COMPAT so it won't bite us in the future. The other patch that
> > > >> fixes ethtool_ops->set_flags compatibility is a bugfix, so it should go
> > > >> in - if we decide that the SFEATURES compatibility should be removed
> > > >> it won't matter.
[...]
> > We could just wait for 2.6.40 and pretend this code part never existed. ;-)
> I think I will make ethtool check for a minimum kernel version of 2.6.40
> before using ETHTOOL_{G,S}FEATURES.

> > I'll rebase the first patch tomorrow. Without it the compatibility in
> > ETHTOOL_SFEATURES for non-converted drivers is busted wrt set_flags.
> This is an improvement, but I still think the fallback is fundamentally
> broken - there's no good way for userland to tell what (if anything)
> went wrong when the return value has ETHTOOL_F_COMPAT set.

The same situation happens with ETHTOOL_F_WISH (userspace needs to reread
the features to find out what happened) and with old ETHTOOL_S{TSO,SG,...}
(those return success if any of the features in the group changes and also
posibly disable other features when one is disabled). This wasn't really
checked before.

Ben, I think I commented on your proposal of the userspace part, but I might
have missed some of your arguments about mine. Let's sum those up:

Your version:
   - reimplements ETHTOOL_Sxx via ETHTOOL_SFEATURES in userspace for kernels
     supporting the latter (note: ETHTOOL_S{SG,...} are not ever going away)
   - causes NETIF_F_* to be an ABI
   - does not support new features

My version:
   - implements only new features via ETHTOOL_SFEATURES (old calls are still used)
   - makes feature names an ABI (for scripts actually, not the tool)
   - supports any new features kernel reports without code changes

Both versions are rough at the edges, but working. Both assume that no
behaviour changes are to be made for old '-K' options.

Best Regards,
Michał Mirosław
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