Subject: [PATCH 2/3] ARM: smp_scu: enable SCU standby support

From: [email-masked] (Will Deacon)
Date: Mon, 21 Jul 2014 10:44:52 +0100

On Mon, Jul 21, 2014 at 10:27:04AM +0100, Shawn Guo wrote:
> On Mon, Jul 21, 2014 at 09:51:50AM +0100, Will Deacon wrote:
> > On Mon, Jul 21, 2014 at 08:45:54AM +0100, Shawn Guo wrote:
> > > With SCU standby enabled, SCU CLK will be turned off when all processors
> > > are in WFI mode.  And the clock will be turned on when any processor
> > > leaves WFI mode.
> > > 
> > > This behavior should be preferable in terms of power efficiency of
> > > system idle.  So let's set the SCU standby bit to enable the support in
> > > function scu_enable().
> > > 
> > > Signed-off-by: Shawn Guo <[email-masked]>
> > > ---
> > >  arch/arm/kernel/smp_scu.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/arm/kernel/smp_scu.c b/arch/arm/kernel/smp_scu.c
> > > index c947508f84e6..9f29d167d02c 100644
> > > --- a/arch/arm/kernel/smp_scu.c
> > > +++ b/arch/arm/kernel/smp_scu.c
> > > @@ -18,6 +18,7 @@
> > >  
> > >  #define SCU_CTRL		0x00
> > >  #define SCU_ENABLE		(1 << 0)
> > > +#define SCU_STANDBY_ENABLE	(1 << 5)
> > >  #define SCU_CONFIG		0x04
> > >  #define SCU_CPU_STATUS		0x08
> > >  #define SCU_INVALIDATE		0x0c
> > > @@ -54,7 +55,7 @@ void scu_enable(void __iomem *scu_base)
> > >  	if (scu_ctrl & SCU_ENABLE)
> > >  		return;
> > >  
> > > -	scu_ctrl |= SCU_ENABLE;
> > > +	scu_ctrl |= SCU_ENABLE | SCU_STANDBY_ENABLE;
> > 
> > I don't think this bit exists on all revisions of the A9.
> 
> Thanks for the info, Will.  Is there any side-effect to write the
> standby bit on those revisions which do not define the bit?

I'm not actually sure what happens at the hardware level, but the TRM is
pretty clear in its definition of `RESERVED':

 `All reserved bits not used by the implementation must be written as 0 and
  read as 0.'

Will