linux-kernel - [PATCH RFC 06/22] drivers: base: Use present CPUs in GENERIC_CPU

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Message-Id: <E1r0JLB-00CTwy-7y@rmk-PC.armlinux.org.uk>
Date:   Tue, 07 Nov 2023 10:29:49 +0000
From:   Russell King (Oracle) <rmk+kernel@...linux.org.uk>
To:     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, linux-csky@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-parisc@...r.kernel.org
Cc:     Salil Mehta <salil.mehta@...wei.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        jianyong.wu@....com, justin.he@....com,
        James Morse <james.morse@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>
Subject: [PATCH RFC 06/22] drivers: base: Use present CPUs in
 GENERIC_CPU_DEVICES

From: James Morse <james.morse@....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().

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
_cpu_up() checks cpu_present().

After this change, only CPUs which are present appear in sysfs.

Signed-off-by: James Morse <james.morse@....com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
---
 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");
 	}
-- 
2.30.2