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    SubjectRe: [PATCH RFC 06/22] drivers: base: Use present CPUs in GENERIC_CPU_DEVICES
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    Hi Russell,

    On 11/7/23 18:29, Russell King (Oracle) wrote:
    > From: James Morse <james.morse@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
    done by cpu_dev_register_generic()? Is the work means register the cpu?

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

    >
    > Of the ACPI architectures that register possible CPUs, arm64 and riscv
    > do not support making possible CPUs present as they use the weak 'always
    > fails' version of arch_register_cpu().
    >
    > Only two of the eight architectures that use GENERIC_CPU_DEVICES have a
    > distinction between present and possible CPUs.
    >
    > The following architectures use GENERIC_CPU_DEVICES but are not SMP,
    > so possible == present:
    > * m68k
    > * microblaze
    > * nios2
    >
    > The following architectures use GENERIC_CPU_DEVICES and consider
    > possible == present:
    > * csky: setup_smp()
    > * processor_probe() sets possible for all CPUs and present for all CPUs
    > except the boot cpu, which will have been done by
    > init/main.c::start_kernel().
    >
    > um appears to be a subarchitecture of x86.
    >
    > The remaining architecture using GENERIC_CPU_DEVICES are:
    > * openrisc and hexagon:
    > where smp_init_cpus() makes all CPUs < NR_CPUS possible,
    > whereas smp_prepare_cpus() only makes CPUs < setup_max_cpus present.
    >
    > 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
    > _cpu_up() checks cpu_present().
    >
    > After this change, only CPUs which are present appear in sysfs.
    >
    > Signed-off-by: James Morse <james.morse@arm.com>
    > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
    > Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

    Thanks,
    Shaoqin

    > ---
    > drivers/base/cpu.c | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
    > index 9ea22e165acd..34b48f660b6b 100644
    > --- a/drivers/base/cpu.c
    > +++ b/drivers/base/cpu.c
    > @@ -533,7 +533,7 @@ static void __init cpu_dev_register_generic(void)
    > #ifdef CONFIG_GENERIC_CPU_DEVICES
    > int i;
    >
    > - for_each_possible_cpu(i) {
    > + for_each_present_cpu(i) {
    > if (register_cpu(&per_cpu(cpu_devices, i), i))
    > panic("Failed to register CPU device");
    > }

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