[dpdk-dev] Explanation of the QoS offset values used in the QoS scheduler example app.

[Aws Ismail <aws.ismail@gmail.com>]

Hi everyone,

I am looking at this portion of the code in the app_thread.c file of the
QoS scheduler example application:

/*
* QoS parameters are encoded as follows:
* Outer VLAN ID defines subport
* Inner VLAN ID defines pipe
* Destination IP 0.0.XXX.0 defines traffic class
* Destination IP host (0.0.0.XXX) defines queue
* Values below define offset to each field from start of frame
*/
#define SUBPORT_OFFSET 7
#define PIPE_OFFSET 9
#define TC_OFFSET 20
#define QUEUE_OFFSET 20
#define COLOR_OFFSET 19

static inline int get_pkt_sched(struct rte_mbuf *m, uint32_t *subport,
uint32_t *pipe, uint32_t *traffic_class, uint32_t *queue, uint32_t *color)
{
uint16_t *pdata = rte_pktmbuf_mtod(m, uint16_t *);

*subport = (rte_be_to_cpu_16(pdata[SUBPORT_OFFSET]) & 0x0FFF) &
(port_params.n_subports_per_port - 1); /* Outer VLAN ID*/

*pipe = (rte_be_to_cpu_16(pdata[PIPE_OFFSET]) & 0x0FFF) &
(port_params.n_pipes_per_subport - 1); /* Inner VLAN ID */

*traffic_class = (pdata[QUEUE_OFFSET] & 0x0F) &
(RTE_SCHED_TRAFFIC_CLASSES_PER_PIPE - 1); /* Destination IP */

*queue = ((pdata[QUEUE_OFFSET] >> 8) & 0x0F) &
(RTE_SCHED_QUEUES_PER_TRAFFIC_CLASS - 1) ; /* Destination IP */

*color = pdata[COLOR_OFFSET] & 0x03; /* Destination IP */

return 0;
}

The offset values do not make sense to me. According to the programmer
guide, the queue selection is SVID/CVID/TC/QID based. And those offset seem
off in this case. Is this because it is assuming that the packet is being
altered before it gets to this stage ?

Can anyone provide a better explanation or at least the reason behind
choosing those offset values shown above.

Thanks.