[coreboot] HT credits and the Opteron

Myles Watson mylesgw at gmail.com
Fri Jul 18 00:39:15 CEST 2008

> -----Original Message-----
> From: Marc Jones [mailto:marc.jones at amd.com]
> Sent: Thursday, July 17, 2008 4:28 PM
> To: Myles Watson
> Cc: 'Coreboot'
> Subject: Re: [coreboot] HT credits and the Opteron
> 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.
> >
> >
> Myles,


Thanks for the quick response.

> I assume that you are talking about the XBAR buffer allocation, 4.6.12?

Sorry. I should have been more clear.  I was talking about 3.3.15: LDTi
Buffer Count Registers.  Does your answer still apply?

> 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.

Now that there is an open-source HT core from the University of Mannheim,
anyone could do this, but maybe most people will leave it alone.


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