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

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Message-ID: <CAGXE3d_=OA+EvX8Q=gELDHSESAQpgt_EavTWGTNLYMnjURLJWw@mail.gmail.com>
Date:	Fri, 16 Dec 2011 10:42:20 +0100
From:	Helmut Schaa <helmut.schaa@...glemail.com>
To:	Dave Taht <dave.taht@...il.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 10:35 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.

Helmut
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