genirq: Remove WARN_ON_ONCE() in generic_handle_domain_irq()

Since commit 0953fb2 ("irq: remove handle_domain_{irq,nmi}()"),
generic_handle_domain_irq() warns if called outside hardirq context, even
though the function calls down to handle_irq_desc(), which warns about the
same, but conditionally on handle_enforce_irqctx().

The newly added warning is a false positive if the interrupt originates
from any other irqchip than x86 APIC or ARM GIC/GICv3.  Those are the only
ones for which handle_enforce_irqctx() returns true.  Per commit
c16816a ("genirq: Add protection against unsafe usage of
generic_handle_irq()"):

 "In general calling generic_handle_irq() with interrupts disabled from non
  interrupt context is harmless. For some interrupt controllers like the
  x86 trainwrecks this is outright dangerous as it might corrupt state if
  an interrupt affinity change is pending."

Examples for interrupt chips where the warning is a false positive are
USB-attached GPIO controllers such as drivers/gpio/gpio-dln2.c:

  USB gadgets are incapable of directly signaling an interrupt because they
  cannot initiate a bus transaction by themselves.  All communication on
  the bus is initiated by the host controller, which polls a gadget's
  Interrupt Endpoint in regular intervals.  If an interrupt is pending,
  that information is passed up the stack in softirq context, from which a
  hardirq is synthesized via generic_handle_domain_irq().

Remove the warning to eliminate such false positives.

Fixes: 0953fb2 ("irq: remove handle_domain_{irq,nmi}()")
Signed-off-by: Lukas Wunner <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Marc Zyngier <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Jakub Kicinski <[email protected]>
CC: Linus Walleij <[email protected]>
Cc: Bartosz Golaszewski <[email protected]>
Cc: Octavian Purdila <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/c3caf60bfa78e5fdbdf483096b7174da65d1813a.1652168866.git.lukas@wunner.de

Author: l1k

kernel/irq/irqdesc.c

@@ -701,7 +701,6 @@ EXPORT_SYMBOL_GPL(generic_handle_irq_safe);
  */
 int generic_handle_domain_irq(struct irq_domain *domain, unsigned int hwirq)
 {
-	WARN_ON_ONCE(!in_hardirq());
 	return handle_irq_desc(irq_resolve_mapping(domain, hwirq));
 }
 EXPORT_SYMBOL_GPL(generic_handle_domain_irq);