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

[Lorenzo Pieralisi]

On Wed, Jan 09, 2013 at 04:17:15PM +0000, Stephen Warren wrote:
> On 01/09/2013 04:34 AM, Lorenzo Pieralisi wrote:
> > On Wed, Jan 09, 2013 at 05:49:46AM +0000, Hiroshi Doyu wrote:
> >> 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?
> > 
> > I think the best solution to this problem consists in /me adding a
> > function, say, arm_dt_nb_cores(), that returns an error if DT probing
> > failed and the number of cores if it succeeded.
> 
> Well, if the primary mechanism needs to be DT-based going forward, I'd
> say just require the DT nodes to be present to use the new function, and
> outright fail if they aren't. That way, everything always works one way.
> It shouldn't be hard to add the CPU nodes for Tegra, right?

Adding /cpu nodes to DT is trivial, but I would like to keep a fall back
mechanism in place for existing platforms to carry out the transition
as smoothly as possible (eg people using new kernels with old DTS).

/cpu nodes will have priority over HW based probing for cores counting.

As discussed with Hiroshi I will post a patch to provide platforms with
a DT cpu map validity check so that the fall back mechanism can be
triggered properly.

Thank you very much,
Lorenzo