IDT PCI Express 89HPES32NT24xG2, User Manual, Page: 91 / 744

IDT Switch Core
PES32NT24xG2 User Manual 4 - 13 January 30, 2013
Notes
Figure 4.4 PCI Express Switch Static Rate Mismatch
The request metering implementation in the switch makes a number of simplifying assumptions that
may or may not be true in all systems. Therefore, it should be expected that some amount of parameter
tuning may be required to achieve optimum performance.
Tuning of the request metering mechanism should take into account the completion
timeout value of the associated requesters (i.e., request metering should be tuned such
that a requester’s completion timeout value is not violated).
Operation
The completion transfer timer is implemented using a counter. The counter is loaded with an estimate of
the number of DWords that will be transferred on the link in servicing the completion and is decremented at
a rate that corresponds to the number of DWords that will be transferred on the link in a 4ns period.
Request metering is enabled on an input port when the Enable (EN) bit is set in the port’s Requester
Metering Control (RMCTL) register.
An non-posted request TLP is allowed to be transferred into the switch core when the request metering
counter is zero.
When a request is transferred into the switch core, the request metering counter is loaded with a value
that estimates the number of DWords associated with the corresponding completion(s). The method for
determining this value is described in section Completion Size Estimation on page 4-14.
The request metering counter is a 24-bit counter. The count represents a fixed-point 0:13:11 number
(i.e., an unsigned number with 13 integer bits and 11 fractional bits) but is treated by the logic as a 24-bit
unsigned integer. The value loaded into the request metering counter for the last non-posted request is
available in the Count (COUNT) field of the Request Metering Counter (RMCOUNT) register.
– The requester metering initial counter value, computed as described in section Completion Size
Estimation on page 4-14, is a fixed point 0:13:3 number.
– The request metering counter is a 24-bit counter that represents a fixed point 0:13:11 number (i.e.,
an unsigned number with 13 integer bits and 11 fractional bits).
– The least significant eight fractional bits of the initial counter value are always implicitly zero.
The request metering counter is decremented by a value that corresponds to the number of DWords
transferred on the link per 4ns period. The value is equal to the sum of the decrement value plus the value
of the Decrement Value Adjustment (DVADJ) field in the RMCTL register.
The decrement value is a fixed-point 0:4:3 number (i.e., an unsigned number with 4 integer bits and 3
fractional bits), determined by the port’s negotiated link width and speed as shown in Table 4.6.The least
significant eight fractional bits of the decrement value are always implicitly zero.
Request
1
Request
2
Request
3
Time
Request
1
Request
2
Request
3
Time
Estimate of Request 1
Completion Transfer Time
Estimate of Request 2
Completion Transfer Time
(a) Request Injection without Request Metering
(b) Request Injection with Request Metering