net/sched: mqprio: allow per-TC user input of FP adminStatus

IEEE 802.1Q-2018 clause 6.7.2 Frame preemption specifies that each packet priority can be assigned to a "frame preemption status" value of either "express" or "preemptible". Express priorities are transmitted by the local device through the eMAC, and preemptible priorities through the pMAC (the concepts of eMAC and pMAC come from the 802.3 MAC Merge layer). The FP adminStatus is defined per packet priority, but 802.1Q clause 12.30.1.1.1 framePreemptionAdminStatus also says that: | Priorities that all map to the same traffic class should be | constrained to use the same value of preemption status. It is impossible to ignore the cognitive dissonance in the standard here, because it practically means that the FP adminStatus only takes distinct values per traffic class, even though it is defined per priority. I can see no valid use case which is prevented by having the kernel take the FP adminStatus as input per traffic class (what we do here). In addition, this also enforces the above constraint by construction. User space network managers which wish to expose FP adminStatus per priority are free to do so; they must only observe the prio_tc_map of the netdev (which presumably is also under their control, when constructing the mqprio netlink attributes). The reason for configuring frame preemption as a property of the Qdisc layer is that the information about "preemptible TCs" is closest to the place which handles the num_tc and prio_tc_map of the netdev. If the UAPI would have been any other layer, it would be unclear what to do with the FP information when num_tc collapses to 0. A key assumption is that only mqprio/taprio change the num_tc and prio_tc_map of the netdev. Not sure if that's a great assumption to make. Having FP in tc-mqprio can be seen as an implementation of the use case defined in 802.1Q Annex S.2 "Preemption used in isolation". There will be a separate implementation of FP in tc-taprio, for the other use cases.

VOVladimir Oltean committed 3 years ago