[coreboot] HT credits and the Opteron

Steve Casselman sc at drccomputer.com
Sun Jul 20 19:52:02 CEST 2008

Credits represent the number of buffers available. There are 7 types of buffers in the Opteron used for the HT subsystem. Each HT link's transmitter sends the receiver on the other side the number of credits (buffers) that are available. If the chip is something like a south bridge then it has a fixed number of credits (buffers) and it sends them to the Opteron.  The Opteron has a pool of buffers that can be shared among the three HT links it has. Their use is mostly in high speed data transfer where you want more data buffers on one port than on another.

As to the CK804 I'm thinking that Dr Lu double checked those settings in the Nvidia data sheet.

The total number of buffers allocated in your BIOS should be checked against the information below.


PS You can see these buffer numbers you have to be root..
lspci -s 0:18.0 -xxx

pcitweak -r 0:18:0 0x90 (reads LDT0 buffer counts)

From the BIOS and Kernel guide...

4.3.15 LDTi Buffer Count Registers

LDT0, LDT1, LDT2 Buffer Count Registers Function 0: Offset 90h, B0h, D0h

Request Buffer Count (Req)—Bits 3–0. 
Posted Request Buffer Count (PReq)—Bits 7–4. 
Response Buffer Count (Rsp)—Bits 11–8. 
Probe Buffer Count (Probe)—Bits 15–12. 
Request Data Buffer Count (ReqD)—Bits 18–16. 
Posted Request Data Buffer Count (PReqD)—Bits 22–20. 
Response Data Buffer Count (RspD)—Bits 26–24. 

4.6.10 XBAR Flow Control Buffers

Table 28. XBAR Input Buffers

Link                      Number of           Number of
                          Command Buffers     Data Buffers

HT Link  0-2                       16 x 3        8 x 3
SRI                                   10            5
MCT                                   12            8
Total                                  70           37

XBAR command and data buffers are independent. There are 70 command buffers available, but a maximum of 64 can be used at any given time. Number of used data buffers is not restricted. The default allocation of command buffers when all HyperTransport links are present is shown in Table 29.

Default Command XBAR Buffers

Link                      Number of           
                          Command Buffers     

HT Link 0-2                       15 x 3      
SRI                                    8     
MCT                                   11     
Total                                 64
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