Re: [PATCH 2/2] CPUFREQ: Mark e_powersaver driver as EXPERIMENTAL and DANGEROUS

[Harald Welte <HaraldWelte@viatech.com>]

On Mon, Jun 08, 2009 at 01:08:46PM +0100, Matthew Garrett wrote:
> On Mon, Jun 08, 2009 at 06:29:36PM +0800, Harald Welte wrote:
> > The e_powersaver driver for VIA's C7 CPU's needs to be marked as
> > DANGEROUS as it configures the CPU to power states that are out
> > of specification.
> > 
> > According to Centaur, all systems with C7 and Nano CPU's support
> > the ACPI p-state method.  Thus, the acpi-cpufreq driver should
> > be used instead.
> 
> Do we know if vendors are actually shipping with the appropriate BIOS 
> tables? 

I strongly assume so, since that is also how the Windows XP driver works (as
I've been told by Centaur).  So if a vendor decides to not ship with the
respective BIOS tables, the power management would fail on Windows, too.

> The number of people using e_powersaver seems to be suspiciously 
> large, though perhaps that's just because the help text implied it was 
> the right choice for C7.

yes, I think that is the case.  I just had a brief look at the driver earlier
today with one of the key Centaur engineers, and he was very clear on it:  Use 
ACPI.

There are also other reasons to use ACPI.  Let's imagine you are having a
system that uses a certain cpu that might clock up to 1600 MHz.  But the system
vendor decides to use the CPU passively cooled and thus wants to restrict the
frequency to 1300MHz.  The standard practise (as recommended by VIA/Centaur BIOS
writers guide) is to remove the > 1300MHz p-states from the ACPI tables.

So even if e_powersaver was fixed to not drive the CPU out of spec, it might
still do that on a particular system/board, where the vendor has decided to 
limit CPU clock for cooling reasons.

-- 
- Harald Welte <HaraldWelte@viatech.com>	    http://linux.via.com.tw/
============================================================================
VIA Free and Open Source Software Liaison