[coreboot] HT credits and the Opteron

Marc Jones marc.jones at amd.com
Fri Jul 18 00:27:33 CEST 2008

Myles Watson wrote:
> Anyone with detailed knowledge of (or more manuals than I have for) the
> Opteron,
> I just noticed that the Nvidia ck804 reduces the amount of credits which the
> Opteron gives it (probably due to an errata.)  Does that mean that there
> should be more credits available for the other HT links?  Is buffer space
> per link or does the CPU share it all between all the links?  What if a link
> is disabled?  Should you give its credits to the others?
> In the BKDG for the Opteron it says that you shouldn't change the values in
> general, because software picks the right values, but that's us.  It doesn't
> say how you should pick new values.

I assume that you are talking about the XBAR buffer allocation, 4.6.12?  
The links are profiled by the designers and then appropriate values are 
assigned. There are some limitations in allocating the buffers so you 
can't just take some form one place an use it somewhere else. For the 
most part, you shouldn't run out of buffers in a typical system, so 
nothing would be gained by adding more to a particular interface.

Generally, we should set what the factory bios sets since those settings 
are validated. If a customer is building their own platforms we will 
work with them to set appropriate values for their silicon.


Marc Jones
Senior Firmware Engineer
(970) 226-9684 Office
mailto:Marc.Jones at amd.com

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