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

["Dumitrescu, Cristian" <cristian.dumitrescu@intel.com>]

Hi,

These are byte offsets used for reading these packet fields, considering that packet bytes are stored in memory in network order, while the CPU is little endian, so byte swapping takes place on read.

This is probably not the best way to write this code, and I agree this portion of the app code is a bit more cryptic than it should be. Using data structures to describe the header format for the input packet (Ethernet/SVLAN/CVLAN/IPv4) and using portable byte swapping macros is probably a better alternative.

This being said, the code implementation, code comments and Sample App Guide description seem to be consistent and correct.

Regards,
Cristian


-----Original Message-----
From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Aws Ismail
Sent: Saturday, February 14, 2015 7:35 PM
To: dev@dpdk.org
Subject: [dpdk-dev] Explanation of the QoS offset values used in the QoS scheduler example app.

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.
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