On Sun, Jul 20, 2008 at 11:52 AM, Steve Casselman <sc(a)drccomputer.com>
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
This was what I was looking for. Since the CK804 uses fewer than the
default number of credits, I was wondering if a coprocessor could take the
"extras" for the link it was on.
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
HT Link 0-2 15 x 3