[PATCH 4/8] ARM: vexpress: use generic cpu idle function for wfi

[Nicolas Pitre]

On Tue, 11 Jun 2013, Pawel Moll wrote:

> On Tue, 2013-06-11 at 06:33 +0100, Sanjay Singh Rawat wrote:
> > >> use cpu_do_idle for entering the wfi mode.
> > >>
> > >> Signed-off-by: Sanjay Singh Rawat <sanjay.rawat at linaro.org>
> > >> ---
> > >>   arch/arm/mach-vexpress/hotplug.c |    3 ++-
> > >>   1 file changed, 2 insertions(+), 1 deletion(-)
> > >>
> > >> diff --git a/arch/arm/mach-vexpress/hotplug.c b/arch/arm/mach-vexpress/hotplug.c
> > >> index f0ce6b8..b3dffc2 100644
> > >> --- a/arch/arm/mach-vexpress/hotplug.c
> > >> +++ b/arch/arm/mach-vexpress/hotplug.c
> > >> @@ -60,7 +60,8 @@ static inline void platform_do_lowpower(unsigned int cpu, int *spurious)
> > >>   	 * code will have already disabled interrupts
> > >>   	 */
> > >>   	for (;;) {
> > >> -		wfi();
> > >> +		/* enter WFI mode */
> > >> +		cpu_do_idle();
> > >>
> > >>   		if (pen_release == cpu_logical_map(cpu)) {
> > >>   			/*
> > >
> > > I probably don't get the whole picture, but may I ask what is the
> > > rationale behind this change? As in: why cpu_do_idle() is better?
> >
> > commit: 8553cb67d2318db327071018fc81084cbabccc46 explains that
> 
> Oh, I've noticed the dsb there, I'm just asking why is it necessary in
> the cpu_die() case? The core is going down anyway, it left the coherency
> domain and we could cut the power now (if only it was possible). All
> cache maintenance (which undoubtedly included dsb) has been long done.
> 
> Now, don't get me wrong - I'm not saying it's wrong to do the dsb. I
> simply would like to know if and why is required (I'm admitting my
> ignorance here ;-) But if you simply grep-ed for "wfi()" in the whole
> kernel and automatically replaced them with "cpu_do_idle()", it doesn't
> make sense.

That's my feeling too.  There is a semantic difference between calling 
cpu_do_idle() and executing a WFI inline.  I'd use cpu_do_idle() only in 
those cases where the call is expected to return and keep the inlined 
WFI otherwise (with the added DSB when necessary which is not all cases 
as explained above).

Nicolas