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

[Hiroshi Doyu]

Lorenzo Pieralisi <lorenzo.pieralisi at arm.com> wrote @ Wed, 9 Jan 2013 19:07:13 +0100:

> 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.

I'll add /cpu nodes for all tegra, anyway.

Until your patch comes, I'll make a code just consider a single core
if DT fails at first. So T114 initial support patch could be merged as
well.