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The 'offline' file in sysfs shows all offline CPUs, including those that aren't present. User-space is expected to remove not-present CPUs from this list to learn which CPUs could be brought online. CPUs can be present but not-enabled. These CPUs can't be brought online until the firmware policy changes, which comes with an ACPI notification that will register the CPUs. With only the offline and present files, user-space is unable to determine which CPUs it can try to bring online. Add a new CPU mask that shows this based on all the registered CPUs. Signed-off-by: James Morse <james.morse@arm.com> Tested-by: Miguel Luis <miguel.luis@oracle.com> Tested-by: Vishnu Pajjuri <vishnu@os.amperecomputing.com> Tested-by: Jianyong Wu <jianyong.wu@arm.com> Acked-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk> Reviewed-by: Gavin Shan <gshan@redhat.com> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> Link: https://lore.kernel.org/r/20240529133446.28446-20-Jonathan.Cameron@huawei.com Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>