IDT PCI Express 89HPES32NT24xG2, User Manual, Page: 92 / 744

IDT Switch Core
PES32NT24xG2 User Manual 4 - 14 January 30, 2013
Notes
The Decrement Value Adjustment (DVADJ) field represents a 1:4:11 number (i.e., a sign-magnitude
fixed-point number with 4 integer bits and 11 fractional bits). The signed nature of the DVADJ field provides
fine grain programmable adjustment of the value by which the counter is decremented.
When the sum of the decrement value plus DVADJ results in a value less than or equal to zero, the
hardware ignores DVADJ and uses the decrement value. The counter stops decrementing when it reaches
zero or when a rollover occurs (i.e., the decrement causes it to become negative).
The computation that occurs on each clock tick by the request metering counter is shown in Figure 4.5.
Figure 4.5 Request Metering Counter Decrement Operation
Completion Size Estimation
This section describes the value that is loaded into the request metering counter when a request is
transferred into the switch core. This value is referred to as the completion size estimate. The completion
size estimate is based on the type of non-posted request as described below.
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 completion size estimate is a 0:13:3
number. The least significant eight fractional bits of the completion size estimate are always implicitly zero.
Non-Posted Writes
The completion size estimate is 0x0018 which corresponds to 3 DWords (3 DWord header).
Link
Width
Link
Speed
Decrement Value Notes
x1 Gen 1 0x02 Corresponds to 1 Byte per clock tick
x2 Gen 1 0x04 Corresponds to 2 Bytes per clock tick
x4 Gen 1 0x08 Corresponds to 4 Bytes per clock tick
x8 Gen 1 0x10 Corresponds to 8 Bytes per clock tick
x1 Gen 2 0x04 Corresponds to 2 Bytes per clock tick
x2 Gen 2 0x08 Corresponds to 4 Bytes per clock tick
x4 Gen 2 0x10 Corresponds to 8 Bytes per clock tick
x8 Gen 2 0x20 Corresponds to 16 Bytes per clock tick
Table 4.6 Request Metering Decrement Value
tmp = RequestMeteringCounter
if ((DecrementValue[LinkSpeed,LinkWidth] + RMCTL.DVADJ) <= 0) {
RequestMeteringCounter -= DecrementValue[LinkSpeed,LinkWidth]
} else {
RequestMeteringCounter -= (DecrementValue[LinkSpeed,LinkWidth] + RMCTL.DVADJ)
}
// prevent negative count
if (tmp < RequestMeteringCounter) {
RequestMeteringCounter = 0
}