irqchip/al-fic: Switch to shared parent interrupt

Until now the driver requested its parent interrupt using the chained IRQ
API (irq_set_chained_handler_and_data()), which only works when each
parent interrupt is wired to a single FIC instance.

A FIC controller is built from groups, each described by its own DT node,
and the groups of one controller share that controller's output line
toward the parent. So a real devicetree has several FIC nodes on one
parent GIC SPI, and a chained handler can only be installed once per
parent. Cascading compounds this: an aggregating group collects several
peripherals' outputs onto the line above it.

To support that, request the parent interrupt as a shared interrupt
(IRQF_SHARED) instead of installing a chained handler. The handler now has
the standard irqreturn_t prototype and reports whether this instance had
anything pending, so the shared-IRQ core can tell which instance on the
line raised the interrupt. IRQF_NO_THREAD is set because the handler only
demultiplexes to the child domain and must not be forced-threaded; all
instances sharing a parent line agree on this flag, as the shared-IRQ core
requires.

The handler reads the group's cause register and returns IRQ_HANDLED when
any unmasked cause bit is set, IRQ_NONE otherwise. That is the signal the
shared-IRQ core needs - "did this instance's hardware raise the line" -
rather than the result of dispatching to the child domain.

For that filter to be right, gc->mask_cache must be valid before the parent
is requested. IRQ_GC_INIT_MASK_CACHE seeds it from the mask register only
on the first child mapping, and never at all for a group with no consumer
in the devicetree; until then the cache reads 0 and every latched cause bit
passes the filter. Seed it from the value al_fic_wire_init() programmed and
drop the flag; nothing can change the register in between, so the read
could only have returned that same value.

request_irq() can fail, unlike irq_set_chained_handler_and_data(), so add
an error path for it. Set IRQ_DOMAIN_FLAG_DESTROY_GC on the domain after
creating it, so irq_domain_remove() tears the generic chips down too and
the single call suffices for both the chip-allocation and request_irq()
failure paths.

Co-developed-by: Talel Shenhar <talel@amazon.com>
Signed-off-by: Talel Shenhar <talel@amazon.com>
Signed-off-by: Eliav Farber <farbere@amazon.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Radu Rendec <radu@rendec.net>
Link: https://patch.msgid.link/20261011050449.41903-6-farbere@amazon.com

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