Re: [RFC PATCH v2 06/35] arm64: setup: Switch over to GENERIC_CPU_DEVICES using arch_register_cpu()

[Jonathan Cameron]

On Thu, 14 Sep 2023 15:07:22 +0100
"Russell King (Oracle)" <linux@xxxxxxxxxxxxxxx> wrote:

> On Thu, Sep 14, 2023 at 12:27:15PM +0100, Jonathan Cameron wrote:
> > On Wed, 13 Sep 2023 16:37:54 +0000
> > James Morse <james.morse@xxxxxxx> wrote:
> >   
> > > To allow ACPI's _STA value to hide CPUs that are present, but not
> > > available to online right now due to VMM or firmware policy, the
> > > register_cpu() call needs to be made by the ACPI machinery when ACPI
> > > is in use. This allows it to hide CPUs that are unavailable from sysfs.
> > > 
> > > Switching to GENERIC_CPU_DEVICES is an intermediate step to allow all
> > > five ACPI architectures to be modified at once.
> > > 
> > > Switch over to GENERIC_CPU_DEVICES, and provide an arch_register_cpu()
> > > that populates the hotpluggable flag. arch_register_cpu() is also the
> > > interface the ACPI machinery expects.
> > > 
> > > The struct cpu in struct cpuinfo_arm64 is never used directly, remove
> > > it to use the one GENERIC_CPU_DEVICES provides.
> > > 
> > > This changes the CPUs visible in sysfs from possible to present, but
> > > on arm64 smp_prepare_cpus() ensures these are the same.
> > > 
> > > Signed-off-by: James Morse <james.morse@xxxxxxx>  
> > 
> > After this the earlier question about ordering of cpu_dev_init()
> > and node_dev_init() is relevant.   
> > 
> > Why won't node_dev_init() call
> > get_cpu_devce() which queries per_cpu(cpu_sys_devices)
> > and get NULL as we haven't yet filled that in?
> > 
> > Or does it do so but that doesn't matter as well create the
> > relevant links later?  
> 
> node_dev_init() will walk through the nodes calling register_one_node()
> on each. This will trickle down to __register_one_node() which walks
> all present CPUs, calling register_cpu_under_node() on each.
> 
> register_cpu_under_node() will call get_cpu_device(cpu) for each and
> will return NULL until the CPU is registered using register_cpu(),
> which will now happen _after_ node_dev_init().
> 
> So, at this point, CPUs won't get registered, and initially one might
> think that's a problem.
> 
> However, register_cpu() will itself call register_cpu_under_node(),
> where get_cpu_device() will return the now populated entry, and the
> sysfs links will be created.
> 
> So, I think what you've spotted is a potential chunk of code that
> isn't necessary when using GENERIC_CPU_DEVICES after this change!
> 

Makes sense thanks. I was just being too lazy to check and bouncing it back
at James! *looks guilty*

Jonathan