Re: skb->priority usage in wireless frame classification

On Fri, Dec 16, 2011 at 10:42 AM, Helmut Schaa
[off-list ref] wrote:

On Fri, Dec 16, 2011 at 10:35 AM, Dave Taht [off-list ref] wrote:

quoted

On Fri, Dec 16, 2011 at 9:45 AM, Helmut Schaa
[off-list ref] wrote:

quoted

[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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