netdev - Re: skb->priority usage in wireless frame classification

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Date:	Fri, 16 Dec 2011 10:35:51 +0100
From:	Dave Taht <dave.taht@...il.com>
To:	Helmut Schaa <helmut.schaa@...glemail.com>
Cc:	netdev@...r.kernel.org,
	linux-wireless <linux-wireless@...r.kernel.org>,
	Johannes Berg <johannes@...solutions.net>
Subject: Re: skb->priority usage in wireless frame classification

On Fri, Dec 16, 2011 at 9:45 AM, Helmut Schaa
<helmut.schaa@...glemail.com> wrote:
> [Adding netdev]
>
> Maybe someone from the netdev people can shed some light on this.
> We've got the following code in net/wireless/util.c for frame classification
> (cfg80211_classify8021d):
>
> /* skb->priority values from 256->263 are magic values to
> * directly indicate a specific 802.1d priority.  This is used
> * to allow 802.1d priority to be passed directly in from VLAN
> * tags, etc.
> */
> if (skb->priority >= 256 && skb->priority <= 263)
>                  return skb->priority - 256;
>
> I was just thinking about making use of this for some internal mac80211
> QoS handling. But is this code still valid? At least I haven't found anything
> in the 802.1q code :(

I made use of the above 'feature' while prototyping some Diffserv work
with iptables. I have no idea if there are users of this obscure
overloading of priority in userspace....

The vlan code at least used to do something like this:

net/8021q/vlan.h: *	@vlan_qos: vlan priority: (skb->priority << 13) & 0xE000

But I don't see that at first glance in the current code... this part
is functionally the same...

vlan_netlink.c:				m.to   = (pm->vlan_qos >> 13) & 0x7;
vlanproc.c:				   mp->priority, ((mp->vlan_qos >> 13) & 0x7));

Assuming that somewhere else was an assignment from priority to this...

> Thanks,
> Helmut
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Dave Täht
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