[v2 3/9] ARM: tegra: # of CPU cores detection w/ & w/o HAVE_ARM_SCU

[Hiroshi Doyu]

Hi,

Stephen Warren <swarren at wwwdotorg.org> wrote @ Tue, 8 Jan 2013 20:32:32 +0100:

> On 01/08/2013 09:21 AM, Mark Rutland wrote:
> > On Tue, Jan 08, 2013 at 02:53:42PM +0000, Hiroshi Doyu wrote:
> >> Mark Rutland <mark.rutland at arm.com> wrote @ Tue, 8 Jan 2013 15:28:28 +0100:
> >>> On Tue, Jan 08, 2013 at 12:47:37PM +0000, Hiroshi Doyu wrote:
> >>>> The method to detect the number of CPU cores on Cortex-A9 MPCore and
> >>>> Cortex-A15 MPCore is different. On Cortex-A9 MPCore we can get this
> >>>> information from the Snoop Control Unit(SCU). On Cortex-A15 MPCore we
> >>>> have to read it from the system coprocessor(CP15), because the SCU on
> >>>> Cortex-A15 MPCore does not have software readable registers. This
> >>>> patch selects the correct method at runtime based on the CPU ID.
> ...
> >>>>  static void __init tegra_smp_init_cpus(void)
> >>>>  {
> >>>> -	unsigned int i, ncores = scu_get_core_count(scu_base);
> >>>> +	unsigned int i, cpu_id, ncores;
> >>>> +	u32 l2ctlr;
> >>>> +	phys_addr_t pa;
> >>>> +
> >>>> +	cpu_id = read_cpuid(CPUID_ID) & CPU_MASK;
> >>>> +	switch (cpu_id) {
> >>>> +	case CPU_CORTEX_A15:
> >>>> +		asm("mrc p15, 1, %0, c9, c0, 2\n" : "=r" (l2ctlr));
> >>>> +		ncores = ((l2ctlr >> 24) & 3) + 1;
> >>>> +		break;
> >>>
> >>> [...]
> >>>
> >>> As mentioned last time [1], you should get this information from the dt
> >>> instead.
> >>
> >> Most of platsmp.c:.smp_init_cpus() implementations seem just to
> >> overwrite # of cores by SCU/MRC detection. Is there any implementation
> >> to use the DT's # and skip SCU/MRC detection in .smp_init_cpus()?
> > 
> > As far as I can see, there's no other platform which just relies on
> > arm_dt_init_cpu_maps. Until recently, it didn't exist, so that makes some
> > sense. As far as I can see, for the Tegra 114 you only need your smp_init_cpus
> > to call set_smp_cross_call(gic_raise_softirq). Everything else you do seems to
> > be handled by arm_dt_init_cpus.

True.

> > I think the best option would be to have a separate smp_ops for your dt
> > platforms where we know cpu nodes are populated (e.g. Tegra 114), where
> > smp_init_cpus is different to that for non-dt platforms. That way non dt
> > platforms can keep the SCU hack for now, and won't be broken, and the dt
> > platforms are far removed from the SCU hack and just use common
> > infrastructure.
>
> Tegra doesn't have any non-DT support now.

What about falling down to SCU/MRC hack only when DT detection fails
or no /cpus entry in DT?

Can arm_dt_init_cpu_maps() return if it suceeds or not?

> 
> If we're going to make Tegra114 read the CPU information from DT, then
> I'd rather just switch over all Tegra platforms to requiring CPU nodes
> in the DT. We have few enough SoC variants that it should be easy to
> switch everything to DT before adding Tegra114 support without causing
> any problems.

Possible.

> But that all said, I'm not convinced it's a good idea to force the
> information to be present in DT when it's just duplicating stuff that
> can be runtime-probed from the HW...

That's also my concern somewhat.

The main purpose to DT CPU core detection is for multiple CPU
clusters. SCU/MRC cannot know anything outside of their own
clusters. In tegra, we have 4 Cortex-A9/A15 core plus 1 shadow
core. Switching those clusters is done transparently. The last A9/A15
core is transparently switched back and forth to the shadow one. So in
this case, SCU/MRC detection seems to work fine.