Revert commit 7a8c994cbb2d ("ACPI: processor: idle: Optimize ACPI idle
driver registration") because it is reported to introduce a cpuidle
regression leading to a kernel crash on a platform using the ACPI idle
driver.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reported-by: Borislav Petkov <bp@alien8.de>
Tested-by: Borislav Petkov (AMD) <bp@alien8.de>
Closes: https://lore.kernel.org/lkml/20251124200019.GIaSS5U9HhsWBotrQZ@fat_crate.local/

Revert commit 5020d05b3476 ("ACPI: processor: Remove unused empty stubs
of some functions") because it depends on a problematic one.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Revert commit bdf780fbcef5 ("ACPI: processor: idle: Rearrange declarations
in header file") because it depends on a problematic one.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Revert commit fbd401e95e56 ("ACPI: processor: idle: Redefine two
functions as void") because it depends on a problematic one.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Revert commit 559f2eacc8a2 ACPI: processor: Do not expose global variable
acpi_idle_driver" because it depends on a problematic one.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Move the cpuidle driver check from __acpi_processor_start() to
acpi_processor_power_init() which allows variable acpi_idle_driver to
become static.

No intentional functional impact.

Signed-off-by: Huisong Li <lihuisong@huawei.com>
Link: https://patch.msgid.link/20250923032428.2656329-2-lihuisong@huawei.com
[ rjw: Subject tweak, new changelog, adjustment of a new comment ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Notice that acpi_processor_power_init() and acpi_processor_power_exit()
don't need to return any values because their callers don't check them
anyway, so redefine those functions as void.

While at it, rearrange the code in acpi_processor_power_init() to
reduce the indentation level, get rid of a redundant local variable
in that function, and rephrase a code comment in it.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>

Group all of the declarations of functions that belong to the ACPI
processor idle driver together in one place in processor.h.

While at it, drop the unnecessary extern modifier from the declaraions
of two functions.

Signed-off-by: Huisong Li <lihuisong@huawei.com>
Link: https://patch.msgid.link/20250911112408.1668431-3-lihuisong@huawei.com
[ rjw: Subject and changelog rewrite ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Empty stubs are defined in processor.h for some functions provided by
the ACPI processor idle driver, but those functions are only used in
the main ACPI processor driver which requires the ACPI processor idle
driver to be present (selecting CONFIG_ACPI_PROCESSOR causes
CONFIG_ACPI_PROCESSOR_IDLE to be selected too automatically).

This means that the empty stubs in question are not really necessary and
if both CONFIG_ACPI_PROCESSOR and CONFIG_ACPI_PROCESSOR_IDLE are unset,
the compiler complains that they are defined, but not used.  Drop them
to get rid of the compiler warning.

Signed-off-by: Huisong Li <lihuisong@huawei.com>
Link: https://patch.msgid.link/20250911112408.1668431-2-lihuisong@huawei.com
[ rjw: Subject and changelog rewrite ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

If CONFIG_ACPI_PROCESSOR_IDLE=n, acpi_processor_register_idle_driver()
and acpi_processor_unregister_idle_driver() are never used and the
empty stubs of them are not needed.  Moreover, they cause the compiler
to complain [1], so remove them.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508300519.tZQHY6HA-lkp@intel.com/ [1]
Fixes: 7a8c994cbb2d ("ACPI: processor: idle: Optimize ACPI idle driver registration")
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Link: https://patch.msgid.link/20250905081900.663869-2-lihuisong@huawei.com
[ rjw: Changelog rewrite ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>