linux-kernel - Re: [RFC PATCH v2 06/35] arm64: setup: Switch over to GENERIC_CPU_DEVICES using arch_register

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Message-ID: <ZQMTmi7blj/4FpbI@shell.armlinux.org.uk>
Date:   Thu, 14 Sep 2023 15:07:22 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     James Morse <james.morse@....com>, linux-pm@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-acpi@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, kvmarm@...ts.linux.dev,
        x86@...nel.org, Salil Mehta <salil.mehta@...wei.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        jianyong.wu@....com, justin.he@....com
Subject: Re: [RFC PATCH v2 06/35] arm64: setup: Switch over to
 GENERIC_CPU_DEVICES using arch_register_cpu()

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@....com> 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@....com>
> 
> 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!

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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