Re: [PATCH RFC v4 02/15] ACPI: processor: Register all CPUs from acpi_processor_get_info()

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Thu, Feb 15, 2024 at 08:22:29PM +0100, Rafael J. Wysocki wrote:
> On Wed, Jan 31, 2024 at 5:50 PM Russell King <rmk+kernel@armlinux.org.uk> wrote:
> > diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
> > index cf7c1cca69dd..a68c475cdea5 100644
> > --- a/drivers/acpi/acpi_processor.c
> > +++ b/drivers/acpi/acpi_processor.c
> > @@ -314,6 +314,18 @@ static int acpi_processor_get_info(struct acpi_device *device)
> >                         cpufreq_add_device("acpi-cpufreq");
> >         }
> >
> > +       /*
> > +        * Register CPUs that are present. get_cpu_device() is used to skip
> > +        * duplicate CPU descriptions from firmware.
> > +        */
> > +       if (!invalid_logical_cpuid(pr->id) && cpu_present(pr->id) &&
> > +           !get_cpu_device(pr->id)) {
> > +               int ret = arch_register_cpu(pr->id);
> > +
> > +               if (ret)
> > +                       return ret;
> > +       }
> > +
> >         /*
> >          *  Extra Processor objects may be enumerated on MP systems with
> >          *  less than the max # of CPUs. They should be ignored _iff
> 
> This is interesting, because right below there is the following code:
> 
>     if (invalid_logical_cpuid(pr->id) || !cpu_present(pr->id)) {
>         int ret = acpi_processor_hotadd_init(pr);
> 
>         if (ret)
>             return ret;
>     }
> 
> and acpi_processor_hotadd_init() essentially calls arch_register_cpu()
> with some extra things around it (more about that below).
> 
> I do realize that acpi_processor_hotadd_init() is defined under
> CONFIG_ACPI_HOTPLUG_CPU, so for the sake of the argument let's
> consider an architecture where CONFIG_ACPI_HOTPLUG_CPU is set.
> 
> So why are the two conditionals that almost contradict each other both
> needed?  It looks like the new code could be combined with
> acpi_processor_hotadd_init() to do the right thing in all cases.
> 
> Now, acpi_processor_hotadd_init() does some extra things that look
> like they should be done by the new code too.
> 
> 1. It checks invalid_phys_cpuid() which appears to be a good idea to me.
> 
> 2. It uses locking around arch_register_cpu() which doesn't seem
> unreasonable either.
> 
> 3. It calls acpi_map_cpu() and I'm not sure why this is not done by
> the new code.
> 
> The only thing that can be dropped from it is the _STA check AFAICS,
> because acpi_processor_add() won't even be called if the CPU is not
> present (and not enabled after the first patch).
> 
> So why does the code not do 1 - 3 above?

Honestly, I'm out of my depth with this and can't answer your
questions - and I really don't want to try fiddling with this code
because it's just too icky (even in its current form in mainline)
to be understandable to anyone who hasn't gained a detailed knowledge
of this code.

It's going to require a lot of analysis - how acpi_map_cpuid() behaves
in all circumstances, what this means for invalid_logical_cpuid() and
invalid_phys_cpuid(), what paths will be taken in each case. This code
is already just too hairy for someone who isn't an experienced ACPI
hacker to be able to follow and I don't see an obvious way to make it
more readable.

James' additions make it even more complex and less readable.

-- 
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