Date: Wed, 8 Oct 2014 15:00:06 +0200
From: Stanislaw Gruszka <sgruszka@redhat.com>
To: Mark Asselstine <asselsm@gmail.com>
Cc: Helmut Schaa <helmut.schaa@googlemail.com>,
	rt2x00 Users List <users@rt2x00.serialmonkey.com>,
	linux-wireless <linux-wireless@vger.kernel.org>
Subject: Re: [PATCH] rt2x00: rt2x00queue: avoid using more headroom then
 driver requested
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On Wed, Oct 08, 2014 at 07:46:33AM -0400, Mark Asselstine wrote:
> > If rt2x00 does not remove the alignment from the frame before giving it back
> > to mac80211 and the same frame comes into rt2x00 again it should be correctly
> > aligned and no additional header space is required. So this should be fine.
> 
> Then I would say this definitely hints at a design flaw in
> rt2x00queue_insert_l2pad(). Take the following scenario.
> 
> * skb's first arrival in rt2x00queue_insert_l2pad(), 3 bytes needed
> for frame alignment, 2 bytes for l2pad results in 3 bytes of headroom
> taken.
Not quite realistic assumption - header length will have to be odd then.

But if such situation would happen we will have:

header_align=2, payload_align=3, l2pad=3

Since payload_align will be bigger than header_align, header_align will
be increased to 6.

Header will be moved by 6 bytes, frame will be moved by 3 bytes,
between header and frame there will be l2pad equal to 3.

> * rt2x00lib_txdone() returns 2 bytes of headroom
Return 3 bytes.

> * skb's second arrival in rt2x00queue_insert_l2pad(), 0 bytes needed
> for frame alignment, 2 bytes for l2pad results in 4 bytes of headroom
> taken.
Header will be moved by 3 bytes.

> * rt2x00lib_txdone() returns 2 bytes of headroom
Return 3 bytes.

> Basically as long as any bytes are required for l2pad the headroom
> will lose 4 bytes again and again, never being returned by
> rt2x00lib_txdone().

I think that's not true - you made a few mistakes in your scenario,
but perhaps I'm wrong :-)

Stanislaw