To support virtual CPU hotplug, ACPI has added an 'online capable' bit
to the MADT GICC entries. This indicates a disabled CPU entry may not
be possible to online via PSCI until firmware has set enabled bit in
_STA.

This means that a "usable" GIC redistributor is one that is marked as
either enabled, or online capable. The meaning of the
acpi_gicc_is_usable() would become less clear than just checking the
pair of flags at call sites. As such, drop that helper function.
The test in gic_acpi_match_gicc() remains as testing just the
enabled bit so the count of enabled distributors is correct.

What about the redistributor in the GICC entry? ACPI doesn't want to say.
Assume the worst: When a redistributor is described in the GICC entry,
but the entry is marked as disabled at boot, assume the redistributor
is inaccessible.

The GICv3 driver doesn't support late online of redistributors, so this
means the corresponding CPU can't be brought online either.
Rather than modifying cpu masks that may already have been used,
register a new cpuhp callback to fail this case. This must run earlier
than the main gic_starting_cpu() so that this case can be rejected
before the section of cpuhp that runs on the CPU that is coming up as
that is not allowed to fail. This solution keeps the handling of this
broken firmware corner case local to the GIC driver. As precise ordering
of this callback doesn't need to be controlled as long as it is
in that initial prepare phase, use CPUHP_BP_PREPARE_DYN.

Systems that want CPU hotplug in a VM can ensure their redistributors
are always-on, and describe them that way with a GICR entry in the MADT.
Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Miguel Luis <miguel.luis@oracle.com>
Co-developed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240529133446.28446-15-Jonathan.Cameron@huawei.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>