Subject: Re: [PATCH] Add idle power save for ppc 4xx

From: Jerone Young <[email-masked]>
Date: Mon, 31 Mar 2008 13:23:46 -0500

On Mon, 2008-03-31 at 12:07 -0500, Josh Boyer wrote:
> On Mon, 2008-03-31 at 08:12 -0500, Jerone Young wrote:
> > # HG changeset patch
> > # User Jerone Young <[email-masked]>
> > # Date 1206969060 18000
> > # Node ID 10aea37177130bbe5de7bee6ec06d9010bc5da1f
> > # Parent  1506aa38ddabb0bf73fff3ac3f3db5f9ef6458cc
> > Add idle power save for ppc 4xx
> > 
> > This patch sets the wait state MSR when power_save is called in cpu_idle loop for ppc4xx. This is mainly to help out virtualization solutions such as KVM. This way the virtualization soultions are able to tell if the guest kernel is idle.
> > 
> > I have tested this on hardware & KVM virtual guest.
> 
> I'm not overly thrilled with adding this to all of 4xx.  It doesn't
> actually save much power at all (1% on a project that actually measured
> it with an amp meter recently) and there's really no other benefit to
> doing it outside of the virtual guest case.
> 
> I'm assuming you pass a dtb to the virtual guest when you start it up.
> Could you define a property in the CPU node there that can be parsed to
> use the power_save function instead of always making it the default?
> 
> > Signed-off-by: Jerone Young <[email-masked]>
> > 
> > diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
> > --- a/arch/powerpc/kernel/Makefile
> > +++ b/arch/powerpc/kernel/Makefile
> > @@ -39,6 +39,7 @@ obj-$(CONFIG_GENERIC_TBSYNC)	+= smp-tbsy
> >  obj-$(CONFIG_GENERIC_TBSYNC)	+= smp-tbsync.o
> >  obj-$(CONFIG_CRASH_DUMP)	+= crash_dump.o
> >  obj-$(CONFIG_6xx)		+= idle_6xx.o l2cr_6xx.o cpu_setup_6xx.o
> > +obj-$(CONFIG_4xx)		+= idle_4xx.o
> >  obj-$(CONFIG_TAU)		+= tau_6xx.o
> >  obj-$(CONFIG_HIBERNATION)	+= swsusp.o suspend.o \
> >  				   swsusp_$(CONFIG_WORD_SIZE).o
> > diff --git a/arch/powerpc/kernel/idle_4xx.c b/arch/powerpc/kernel/idle_4xx.c
> > new file mode 100644
> > --- /dev/null
> > +++ b/arch/powerpc/kernel/idle_4xx.c
> 
> Can this be added to sysdev/ppc4xx_soc.c instead?

Probably. Though the other platforms have there power_save code in
idle_<platform>.S files, as they are in assembly.

Also I don't appear to have ppc4xx_soc.c in my source (using
2.6.25-rc6).

> 
> > +#include <asm/processor.h>
> > +#include <asm/machdep.h>
> > +
> > +void ppc4xx_idle()
> > +{
> > +	unsigned long msr_save;
> > +
> > +	/* set wait state MSR */
> > +	local_irq_enable();
> > +	msr_save = mfmsr();
> > +	mtmsr(msr_save|MSR_WE);
> > +	local_irq_disable();
> > +}
> 
> I agree with Hollis on both the MSR_WE|MSR_EE and removing the
> local_irq_disable changes.

You mention adding MSR_CE in a followup email. I'll add that to and
remove local_irq_disable.

> 
> > diff --git a/arch/powerpc/kernel/setup_32.c b/arch/powerpc/kernel/setup_32.c
> > --- a/arch/powerpc/kernel/setup_32.c
> > +++ b/arch/powerpc/kernel/setup_32.c
> > @@ -132,6 +132,10 @@ void __init machine_init(unsigned long d
> >  	if (cpu_has_feature(CPU_FTR_CAN_DOZE) ||
> >  	    cpu_has_feature(CPU_FTR_CAN_NAP))
> >  		ppc_md.power_save = ppc6xx_idle;
> > +#endif
> > +
> > +#ifdef CONFIG_4xx
> > +	ppc_md.power_save = ppc4xx_idle;
> >  #endif
> 
> I agree this belongs in platform setup code.  Finding the right spot for
> it might be a bit of a challenge.
> 
> josh
>