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

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Date:	Fri, 16 Dec 2011 10:53:43 +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 10:42 AM, Helmut Schaa
<helmut.schaa@...glemail.com> wrote:
> On Fri, Dec 16, 2011 at 10:35 AM, Dave Taht <dave.taht@...il.com> wrote:
>> 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
>
> Yeah, I saw that too but +256 would be << 8 and not << 13.

Exactly. It's why I used the word 'overload'. Someone in the distant
past saw >> 8 was 'reserved', and figured >> 13 was a safe place to
stick some valuable bits...

or vice versa.

> Helmut



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Dave Täht
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