4. Internal Switching Fabric > Packet Queuing
92
Tsi578 User Manual
June 6, 2016
Integrated Device Technology
www.idt.com
4.4.1.1
Egress Watermark
The ISF egress arbiter generates flow control for a given priority of traffic based on
watermarks
.
Watermarks are defined for priority 0, 1, and 2 packets (no watermark is defined for priority 3 packets
because they are always accepted whenever there are free buffers). The watermark values are
programmable and are located in
“RapidIO Port x ISF Watermarks” on page 397
Rules for Programming Watermarks
The following rules are applied to Tsi578 watermarks:
•
No watermark is associated with Priority 3 packets
•
A priority
x
packet is accepted in the buffer if the number of free buffers is greater than the
programmed watermark of the associated priority. For example, when the PRIO1WM field is
programmed to three, a priority 1 packet is accepted only when there are four or more free buffers.
•
The three programmed watermarks (PRIO0WM, PRIO1WM, and PRIO2WM) must contain
values where PRIO0WM > PRIO1WM > PRIO2WM > 0 at all times.
•
The watermarks for the three priorities must allow for the following minimum levels:
— PRIO2WM >= 1
— PRIO1WM >= 2
— PRIO0WM >= 3
The hierarchy of watermarks ensures that packets of lower priority can never consume all buffers and
prevent packets of higher priority from being transmitted. With the correct setting of the watermarks,
there is at least one priority 3 packet in a full buffer (that is, if all buffers are filled, then at least one of
the buffers must be occupied by a priority 3 packet).
Violating any one of the watermark rules creates a deadlock situation in the system.