Egress Buffer; Table 3.2 Efb Buffer Sizes - Renesas IDT 89HPES48H12G2 User Manual

IDT Switch Core
Notes
PES48H12G2 User Manual
Port IFB
Mode Queue
x8 Posted
Merged
Non Posted
Completion

Egress Buffer

The EFBs provide head-of-line-blocking (HOLB) relief to the IFBs by allowing packets to be stored in an
egress port's EFB even if the port's link does not have sufficient credits to accept the packet. HOLB relief
allows subsequent packets in the IFB to be transferred to their destinations efficiently. As packets are trans-
ferred from an IFB to an EFB, they are subjected to the egress port's TC/VC mapping and stored in the
EFB.
Each EFB consists of three queues. These are the posted queue, non-posted queue, and completion
queue. The use of these queues allows for packet re-ordering to improve transmission efficiency on the
egress link. Refer to section Packet Ordering on page 3-4 for details.
– The queues for both EFBs are implemented using a descriptor memory and a data memory.
– When two x4 ports are merged to create a x8 port, the descriptor and data memories for both x4
ports are merged.
The default size of each of these queues is shown in Table 3.2.
Port
Mode
x4
Bifurcated
x8
Merged
In addition to providing HOLB relief, the EFB is used as a dynamically sized replay buffer. This allows for
efficient use of the egress buffer space: when transmitted packets are not being acknowledged by the link
partner the replay buffer grows to allow further transmission; when transmitted packets are successfully
acknowledged by the link partner the replay buffer shrinks and this space is used as egress buffer space to
provide maximum HOLB relief to the IFBs. Assuming a link partner issues acknowledges at the rates
recommended in the PCI Express 2.0 spec, the replay buffer naturally grows to the optimal size for the
port's link width and speed. Table 3.3 shows the maximum number of TLPs that may be stored in the EFB's
replay buffer.
Total Size and
Limitations
(per-port)
12352 Bytes and up to 127
TLPs
2048 Bytes and up to 127
TLPs
12352 Bytes and up to 127
TLPs
Table 3.1 IFB Buffer Sizes (Part 2 of 2)
EFB Total Size and Limitations
Queue (per-port)
Posted 6176 Bytes and up to 64 TLPs
Non Posted 1024 Bytes and up to 64 TLPs
Completion 6176 Bytes and up to 64 TLPs
Posted 12352 Bytes and up to 128 TLPs
Non Posted 2048 Bytes and up to 128 TLPs
Completion 12352 Bytes and up to 128 TLPs

Table 3.2 EFB Buffer Sizes

3 - 2
Advertised Advertised
Data Header
Credits Credits
772 127
128 127
772 127
April 5, 2013