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

[Shaoqin Huang]

On 11/9/23 18:29, Russell King (Oracle) wrote:
> 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.
> 

Oh, Sorry for that.

>> 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.
> 

Thanks for your great explanation. Change commit message to this makes 
me understand well.

Thanks,
Shaoqin

>>> 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.
>