Re: C states on AMD SMP

Erik Slagter
2005-09-11T10:09:00Z

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On Sat, 2005-09-10 at 19:08 +0200, Pavel Troller wrote:

> > a) mostly for the amd760 chipset (as opposed to amd768)

> But which chipset is in my machine, when for example lspci shows:
> 00:00.0 Host bridge: Advanced Micro Devices [AMD] AMD-760 MP [IGD4-2P] System Controller (rev 11)
> 00:01.0 PCI bridge: Advanced Micro Devices [AMD] AMD-760 MP [IGD4-2P] AGP Bridge
> 00:07.0 ISA bridge: Advanced Micro Devices [AMD] AMD-768 [Opus] ISA (rev 04)
> 00:07.1 IDE interface: Advanced Micro Devices [AMD] AMD-768 [Opus] IDE (rev 04)
> Is it a compbination of 760/768 ? If the important part is in the first line,
> does it mean that it cannot work that simple as a "pure" 768 ?

That actually is a amd768 chipset. The southbridge (in your case 768) is
what gives the chipset the name (btw I have exactly the same config).
The northbridge is always 760.

> > b) some registers are already set by default or by bios
> But when the bios forbids PM, takes it enough care to put them to the correct
> state ?

I can change it in my bios settings.

> > c) some registers enable some extra savings
> Another reason for a specialized driver.

Ok, I guess you are right ;-) The default acpi driver lets one CPU go to
C3, one only to C1, and it looks like there actually doesn't happen
anything.

So I guess I will try amd76x_pm another try. Advantage is that I don't
use my saa7134 (tv capturing with extensive bus mastering) anymore, so
the busmastering latency isn't much an issue anymore.

Guess I'll have to give up the idea of plain acpi idling on SMP anyway.

> Probably not, but for example the previous version of the driver with C3 enabled
> (the current version seems not to contain C3 support) entered C3 in my case
> regularly (less than C2, but my machine is running a lot of daemons keeping CPU
> load at 0.2 or more so maybe it didn't had enough time to enter it so often).

The current version has an "extra" patch to add C3/NTH/POS support. The
latter two are imho actually not useful. I never got throttling working
(also by taking the except steps as described in the chipset docs), so I
guess it's broken; it won't buy you much power savings anyway. POS off
course is totally useless on a server system.