linux-kernel - [RFC PATCH 03/32] drivers: base: Use present CPUs in GENERIC_CPU

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Date:   Fri,  3 Feb 2023 13:50:14 +0000
From:   James Morse <james.morse@....com>
To:     linux-pm@...r.kernel.org, loongarch@...ts.linux.dev,
        kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
        linux-acpi@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, x86@...nel.org
Cc:     Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Borislav Petkov <bp@...en8.de>, H Peter Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Huacai Chen <chenhuacai@...nel.org>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Len Brown <lenb@...nel.org>,
        Rafael Wysocki <rafael@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        Salil Mehta <salil.mehta@...wei.com>,
        Russell King <linux@...linux.org.uk>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: [RFC PATCH 03/32] drivers: base: Use present CPUs in GENERIC_CPU_DEVICES

The four ACPI architectures only create sysfs entries using register_cpu()
for present CPUs, whereas GENERIC_CPU_DEVICES does this for possible CPUs.

Only two of the eight architectures that use GENERIC_CPU_DEVICES have a
distinction between present and possible CPUs.

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

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()
 * hexagon: compare smp_start_cpus() and smp_prepare_cpus()
 * parisc: smp_prepare_boot_cpu() marks the boot cpu as present,
   processor_probe() sets possible for all CPUs and present for all CPUs
   except the boot cpu.

um appears to be a subarchitecture of x86.

The remaining architecture using GENERIC_CPU_DEVICES is openrisc,
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 systems that boot with max_cpus=1 would not see other
CPUs present in sysfs. This should not be a problem as these CPUs can't
be brought online as _cpu_up() checks cpu_present().

Signed-off-by: James Morse <james.morse@....com>
---
 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 4c98849577d4..cf6407c34ede 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -500,7 +500,7 @@ 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