netdev - Re: [PATCH v2] net: set default_ethtool_ops in register

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Message-Id: <20130110.021202.232771737304117743.davem@davemloft.net>
Date:	Thu, 10 Jan 2013 02:12:02 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	sgruszka@...hat.com
Cc:	netdev@...r.kernel.org, edumazet@...gle.com,
	greearb@...delatech.com, bjorn@...k.no,
	linux-wireless@...r.kernel.org, bhutchings@...arflare.com,
	mirqus@...il.com
Subject: Re: [PATCH v2] net: set default_ethtool_ops in register_netdevice

From: David Miller <davem@...emloft.net>
Date: Thu, 10 Jan 2013 02:07:29 -0800 (PST)

> From: Stanislaw Gruszka <sgruszka@...hat.com>
> Date: Thu, 10 Jan 2013 09:36:42 +0100
> 
>> On Wed, Jan 09, 2013 at 11:57:38PM -0800, David Miller wrote:
>>> From: Stanislaw Gruszka <sgruszka@...hat.com>
>>> Date: Tue, 8 Jan 2013 16:38:51 +0100
>>> 
>>> > Since:
>>> > 
>>> > commit 2c60db037034d27f8c636403355d52872da92f81
>>> > Author: Eric Dumazet <edumazet@...gle.com>
>>> > Date:   Sun Sep 16 09:17:26 2012 +0000
>>> > 
>>> >     net: provide a default dev->ethtool_ops
>>> > 
>>> > wireless core does not correctly assign ethtool_ops. In order to fix
>>> > the problem, move assignement of default_ethtool_ops to
>>> > register_netdevice(). This is safe because both register_netdevice()
>>> > and dev_ethtool() are protected by RTNL lock.
>>> > 
>>> > Patch is besed on hint of Micha┅ Miros┅aw.
>>> > 
>>> > Signed-off-by: Stanislaw Gruszka <sgruszka@...hat.com>
>>> > Cc: stable@...r.kernel.org # 3.7+
>>> > ---
>>> > v1 -> v2: change order of default_ethtool_ops initialization to avoid
>>> > the problem. Change the subject accordingly.
>>> 
>>> I don't understand this.  Why is the assignment of default_ethtool_ops
>>> at netdev allocation time not working?  Is wireless really not using
>>> alloc_netdev*()?
>> 
>> It does. This is done on individual cfg80211 drivers , i.e. on mac80211
>> or full mac drivers. After alloc_netdev*() call, some cfg80211 drivers
>> provide they own ethtool_ops, but some do not. For them, wireless core
>> provide generic cfg80211_ethtool_ops, which is assigned in
>> NETDEV_REGISTER notify call: 
>> 
>> 	if (!dev->ethtool_ops)
>> 		dev->ethtool_ops = &cfg80211_ethtool_ops;
>> 
>> But after Eric's commit, dev->ethtool_ops is no longer NULL (on cfg80211
>> drivers without custom ethtool_ops), but points to &default_ethtool_ops.
> 
> The whole idea is to remove these kinds of NULL tests against
> dev->ethtool_ops, thus creating the invariant that given any netdev
> one must never be able to see a non-NULL value there.

Of course I meant that we should never see a NULL value in
dev->ethtool_ops.

I would suggest fixing this by making the wireless core express it's
intentions, that it wants to use a different default ethtool ops, to
the netdev allocation layer.

You have two way to do this:

1) Add default_ethtool_ops argument to alloc_netdev*().

2) Add a "netdev_set_default_ethtool_ops(netdev, ops)"

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