[PATCH RFC 06/22] drivers: base: Use present CPUs in GENERIC_CPU_DEVICES

[Russell King (Oracle)]

On Thu, Nov 09, 2023 at 06:09:32PM +0800, Shaoqin Huang wrote:
> Hi Russell,
> 
> On 11/7/23 18:29, Russell King (Oracle) wrote:
> > From: James Morse <james.morse at arm.com>
> > 
> > Three of the five ACPI architectures create sysfs entries using
> > register_cpu() for present CPUs, whereas arm64, riscv and all
> > GENERIC_CPU_DEVICES do this for possible CPUs.
> > 
> > Registering a CPU is what causes them to show up in sysfs.
> > 
> > It makes very little sense to register all possible CPUs. Registering
> > a CPU is what triggers the udev notifications allowing user-space to
> > react to newly added CPUs.
> > 
> > To allow all five ACPI architectures to use GENERIC_CPU_DEVICES, change
> > it to use for_each_present_cpu(). Making the ACPI architectures use
> > GENERIC_CPU_DEVICES is a pre-requisite step to centralise their
> > cpu_register() logic, before moving it into the ACPI processor driver.
> > When ACPI is disabled this work would be done by
> > cpu_dev_register_generic().
> 
> What do you actually mean about when ACPI is disabled this work would be

Firstly, please note that "you" is not appropriate here. This is James'
commit message, not mine.

> done by cpu_dev_register_generic()? Is the work means register the cpu?

When ACPI is disabled _and_ CONFIG_GENERIC_CPU_DEVICES is enabled, then
cpu_dev_register_generic() will call arch_register_cpu() for each present
CPU after this commit, rather than for each _possible_ CPU (which is the
actual code change here.)

> I'm not quite understand that, and how about when ACPI is enabled, which
> function do this work?

This is what happens later in the series.

"drivers: base: Allow parts of GENERIC_CPU_DEVICES to be overridden"
adds a test for CONFIG_GENERIC_CPU_DEVICES, so this will only be used
with architectures using GENERIC_CPU_DEVICES. Then in:

"ACPI: processor: Register all CPUs from acpi_processor_get_info()"
which is not part of this series, this adds a call to arch_register_cpu()
in the ACPI code, and disables this path via a test for !acpi_disabled.

Essentially, this path gets used to register the present CPUs when
firmware (ACPI) isn't going to be registering the present CPUs.

I've changed this to:

"It makes very little sense to register all possible CPUs. Registering
a CPU is what triggers the udev notifications allowing user-space to
react to newly added CPUs.

"To allow all five ACPI architectures to use GENERIC_CPU_DEVICES, change
it to use for_each_present_cpu().

"Making the ACPI architectures use GENERIC_CPU_DEVICES is a pre-requisite
step to centralise their register_cpu() logic, before moving it into the
ACPI processor driver. When we add support for register CPUs from ACPI
in a later patch, we will avoid registering CPUs in this path."

which I hope makes it clearer.

> > After this change, openrisc and hexagon systems that use the max_cpus
> > command line argument would not see the other CPUs present in sysfs.
> > This should not be a problem as these CPUs can't bre brought online as
>                                              ^ nit: can't be

Thanks, I'll fix that.

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