netdev - Re: [PATCH 1/2] mac802154: add a software MAC 802.15.4 implementation

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Date:	Wed, 12 Aug 2009 15:13:57 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>
Cc:	netdev@...r.kernel.org, linux-wireless@...r.kernel.org,
	Sergey Lapin <slapin@...fans.org>
Subject: Re: [PATCH 1/2] mac802154: add a software MAC 802.15.4
 implementation

On Wed, 2009-08-12 at 17:06 +0400, Dmitry Eremin-Solenikov wrote:

> > In addition to that, you can't put anything into skb->cb, then push the
> > frame to the master netdev, and expect things in skb->cb to still be
> > there when the frame arrives at the master netdev. Not sure you do that
> > (I hope not because that would be very buggy), but eventually you'll
> > probably find that you do want that, etc.
> 
> Hmm. It works for us. Could you please tell me more about the problems
> with skb->cb ?

Uh, well, you don't own the skb->cb between dev_queue_xmit() and
ndo_start_xmit(). In fact, your phy_cb data will be overwritten by the
qdisc.

So your code is completely and utterly broken just like mac80211 was for
a long time.

Also, looking into this a bit more, I see no reason to allocate a work
struct every time you get a frame -- better just queue them up and stick
everything else into skb->cb. In fact, you don't need a master netdev
here anyway, just move all code from ieee802154_master_hard_start_xmit()
into ieee802154_net_xmit().

johannes

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