Re: [PATCH] irq_bcm2836: Send event when onlining sleeping cores

[Phil Elwell]

On 09/05/2017 19:14, Marc Zyngier wrote:
> On 09/05/17 19:08, Eric Anholt wrote:
> > Marc Zyngier <marc.zyngier@xxxxxxx> writes:
> >
> > > On 09/05/17 17:59, Eric Anholt wrote:
> > > > Phil Elwell <phil@xxxxxxxxxxxxxxx> writes:
> > > >
> > > > > In order to reduce power consumption and bus traffic, it is sensible
> > > > > for secondary cores to enter a low-power idle state when waiting to
> > > > > be started. The wfe instruction causes a core to wait until an event
> > > > > or interrupt arrives before continuing to the next instruction.
> > > > > The sev instruction sends a wakeup event to the other cores, so call
> > > > > it from bcm2836_smp_boot_secondary, the function that wakes up the
> > > > > waiting cores during booting.
> > > > >
> > > > > It is harmless to use this patch without the corresponding change
> > > > > adding wfe to the ARMv7/ARMv8-32 stubs, but if the stubs are updated
> > > > > and this patch is not applied then the other cores will sleep forever.
> > > > >
> > > > > See: https://github.com/raspberrypi/linux/issues/1989
> > > > >
> > > > > Signed-off-by: Phil Elwell <phil@xxxxxxxxxxxxxxx>
> > > > > ---
> > > > >  drivers/irqchip/irq-bcm2836.c | 3 +++
> > > > >  1 file changed, 3 insertions(+)
> > > > >
> > > > > diff --git a/drivers/irqchip/irq-bcm2836.c b/drivers/irqchip/irq-bcm2836.c
> > > > > index e10597c..6dccdf9 100644
> > > > > --- a/drivers/irqchip/irq-bcm2836.c
> > > > > +++ b/drivers/irqchip/irq-bcm2836.c
> > > > > @@ -248,6 +248,9 @@ static int __init bcm2836_smp_boot_secondary(unsigned int cpu,
> > > > >  	writel(secondary_startup_phys,
> > > > >  	       intc.base + LOCAL_MAILBOX3_SET0 + 16 * cpu);
> > > > >
> > > > > +	dsb(sy); /* Ensure write has completed before waking the other CPUs */
> > > > > +	sev();
> > > > > +
> > > > >  	return 0;
> > > > >  }
> > > >
> > > > This is also the behavior that the standard arm64 spin-table method has,
> > > > which we unfortunately can't quite use.
> > >
> > > And why is that so? Why do you have to reinvent the wheel (and hide the
> > > cloned wheel in an interrupt controller driver)?
> > >
> > > That doesn't seem right to me.
> >
> > The armv8 stubs (firmware-supplied code in the low page that do the
> > spinning) do actually implement arm64's spin-table method.  It's the
> > armv7 stubs that use these registers in the irqchip instead of plain
> > addresses in system memory.
>
> Let's put ARMv7 aside for the time being. If your firmware already
> implements spin-tables, why don't you simply use that at least on arm64?

We do.