[PATCH 5/5] powerpc/microwatt: Add SMP support

[Paul Mackerras]

On Wed, Jan 29, 2025 at 06:12:55PM +1000, Nicholas Piggin wrote:
> On Wed Jan 29, 2025 at 4:57 PM AEST, Paul Mackerras wrote:

> > void __init microwatt_init_smp(void)
> > {
> > 	volatile unsigned char __iomem *syscon;
> > 	int ncpus;
> > 	int timeout;
> >
> > 	syscon = early_ioremap(SYSCON_BASE, 0x100);
> 
> ioremap is not up by SMP init time? I always have to
> trawl through init spaghetti to work it out. I guess it's
> early SMP init.

I had a little difficulty with this, and ended up with this function
(microwatt_init_smp) being called at platform probe time, which is why
the early_ioremap.  We need the secondary CPUs spinning in
__secondary_hold before smp_release_cpus() gets called, because that
is what sends them to generic_secondary_smp_init().
Now smp_release_cpus() is called in setup_arch() before even the
platform's setup_arch() function.  (And also, smp_setup_cpu_maps()
gets called even earlier.)  Hence the choice of the platform probe
function.

On platforms with OF and RTAS, the secondary CPUs are started and left
spinning in __secondary_hold by prom_init(), which is right at the
beginning, before the kernel proper starts executing.  Microwatt
doesn't have OF or RTAS, and the secondary CPUs are held in reset
until microwatt_init_smp() releases them (and then they start at 0).
Doing that as early as possible therefore more-or-less matches what
OF/RTAS platforms do.

Paul.