Re: skb->priority usage in wireless frame classification
From: Dave Taht <hidden>
Date: 2011-12-16 09:53:43
Also in:
linux-wireless
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) & 0xE000Yeah, 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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