ACPI: irq: Allow acpi_gsi_to_irq() to have an arch-specific fallback

It appears that the generic version of acpi_gsi_to_irq() doesn't
fallback to establishing a mapping if there is no pre-existing
one while the x86 version does.

While arm64 seems unaffected by it, LoongArch is relying on the x86
behaviour. In an effort to prevent new architectures from reinventing
the proverbial wheel, provide an optional callback that the arch code
can set to restore the x86 behaviour.

Hopefully we can eventually get rid of this in the future once
the expected behaviour has been clarified.

Reported-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Tested-by: Hanjun Guo <guohanjun@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Link: https://lore.kernel.org/r/1658314292-35346-4-git-send-email-lvjianmin@loongson.cn

Author: Marc Zyngier

drivers/acpi/irq.c

@@ -13,6 +13,7 @@
 enum acpi_irq_model_id acpi_irq_model;
 
 static struct fwnode_handle *(*acpi_get_gsi_domain_id)(u32 gsi);
+static u32 (*acpi_gsi_to_irq_fallback)(u32 gsi);
 
 /**
  * acpi_gsi_to_irq() - Retrieve the linux irq number for a given GSI
@@ -32,9 +33,12 @@ int acpi_gsi_to_irq(u32 gsi, unsigned int *irq)
 					DOMAIN_BUS_ANY);
 	*irq = irq_find_mapping(d, gsi);
 	/*
-	 * *irq == 0 means no mapping, that should
-	 * be reported as a failure
+	 * *irq == 0 means no mapping, that should be reported as a
+	 * failure, unless there is an arch-specific fallback handler.
 	 */
+	if (!*irq && acpi_gsi_to_irq_fallback)
+		*irq = acpi_gsi_to_irq_fallback(gsi);
+
 	return (*irq > 0) ? 0 : -EINVAL;
 }
 EXPORT_SYMBOL_GPL(acpi_gsi_to_irq);
@@ -301,6 +305,16 @@ void __init acpi_set_irq_model(enum acpi_irq_model_id model,
 	acpi_get_gsi_domain_id = fn;
 }
 
+/**
+ * acpi_set_gsi_to_irq_fallback - Register a GSI transfer
+ * callback to fallback to arch specified implementation.
+ * @fn: arch-specific fallback handler
+ */
+void __init acpi_set_gsi_to_irq_fallback(u32 (*fn)(u32))
+{
+	acpi_gsi_to_irq_fallback = fn;
+}
+
 /**
  * acpi_irq_create_hierarchy - Create a hierarchical IRQ domain with the default
  *                             GSI domain as its parent.

include/linux/acpi.h

@@ -357,6 +357,7 @@ int acpi_isa_irq_to_gsi (unsigned isa_irq, u32 *gsi);
 
 void acpi_set_irq_model(enum acpi_irq_model_id model,
 			struct fwnode_handle *(*)(u32));
+void acpi_set_gsi_to_irq_fallback(u32 (*)(u32));
 
 struct irq_domain *acpi_irq_create_hierarchy(unsigned int flags,
 					     unsigned int size,