Re: Entering C3 Processor State without a Bus Arbiter

[Nate Lawson <nate-Y6VGUYTwhu0@public.gmane.org>]

On Thu, 15 Jan 2004, Dino Klein wrote:
> I'm curios as to why the implementation does not enter C3 without the
> presence of a Bus Arbiter; also, how come a comment in the source claims
> that C3/WBINVD is too expensive?
>
> After removing the restrictions in the source files, and adding a WBINVD
> instruction right before the read from P_LVL3 - the processor was put in
> the C3 state. This is on my Via Apollo Pro133A based Abit VP6 board,
> which does not have a Bus Arbiter. Just for the record, I did pull out
> the second CPU, so I can do "safer" experimentation.

WBINVD is too expensive when you're entering C3 hundreds of times per
second.  Try playing back an mp3 while dd'ing a CDROM in PIO mode to your
hard drive.  Goodbye data, hello backups!  The CPU can't snoop the bus in
C3.

> I've seen a thread talking about how the processor was not put into C3
> as often enough, when the USB drivers were loaded - in my case I would
> say that it was put too often, this with the UHCI + EHCI + HID drivers
> loaded, and two mice and one hub attached.

Um, because without BM detection, you're entering C3 with BM active while
USB is enabled.

-Nate


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