riscv: Helper to parse hart index

RISC-V APLIC specification defines "hart index" in [1]. Similar definitions
can be found for ACLINT in [2]

Quote from the APLIC specification:

Within a given interrupt domain, each of the domain’s harts has a unique
index number in the range 0 to 2^14 − 1 (= 16,383). The index number a
domain associates with a hart may or may not have any relationship to the
unique hart identifier (“hart ID”) that the RISC-V Privileged
Architecture assigns to the hart. Two different interrupt domains may
employ entirely different index numbers for the same set of harts.

Further, it says in "4.5 Memory-mapped control region for an interrupt
domain":

The array of IDC structures may include some for potential hart index
numbers that are not actual hart index numbers in the domain.  For example,
the first IDC structure is always for hart index 0, but 0 is not
necessarily a valid index number for any hart in the domain.

Support arbitrary hart indices specified in an optional property
"riscv,hart-indexes" which is specified as an array of u32 elements, one
per interrupt target, listing hart indexes in the same order as in
"interrupts-extended".

If this property is not specified, fall back to use logical hart indices
within the domain.

Signed-off-by: Vladimir Kondratiev <vladimir.kondratiev@mobileye.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250612143911.3224046-2-vladimir.kondratiev@mobileye.com
Link: https://github.com/riscv/riscv-aia [1]
Link: https://github.com/riscvarchive/riscv-aclint [2]

2 files changed, 36 insertions, 0 deletions

diff --git a/arch/riscv/include/asm/irq.h b/arch/riscv/include/asm/irq.h
index 7b038f3b7cb0..59c975f750c9 100644
--- a/arch/riscv/include/asm/irq.h
+++ b/arch/riscv/include/asm/irq.h
@@ -22,6 +22,8 @@ void arch_trigger_cpumask_backtrace(const cpumask_t *mask, int exclude_cpu);
 void riscv_set_intc_hwnode_fn(struct fwnode_handle *(*fn)(void));
 
 struct fwnode_handle *riscv_get_intc_hwnode(void);
+int riscv_get_hart_index(struct fwnode_handle *fwnode, u32 logical_index,
+			 u32 *hart_index);
 
 #ifdef CONFIG_ACPI
 
diff --git a/arch/riscv/kernel/irq.c b/arch/riscv/kernel/irq.c
index 9ceda02507ca..b6af20bc300f 100644
--- a/arch/riscv/kernel/irq.c
+++ b/arch/riscv/kernel/irq.c
@@ -32,6 +32,40 @@ struct fwnode_handle *riscv_get_intc_hwnode(void)
 }
 EXPORT_SYMBOL_GPL(riscv_get_intc_hwnode);
 
+/**
+ * riscv_get_hart_index() - get hart index for interrupt delivery
+ * @fwnode: interrupt controller node
+ * @logical_index: index within the "interrupts-extended" property
+ * @hart_index: filled with the hart index to use
+ *
+ * RISC-V uses term "hart index" for its interrupt controllers, for the
+ * purpose of the interrupt routing to destination harts.
+ * It may be arbitrary numbers assigned to each destination hart in context
+ * of the particular interrupt domain.
+ *
+ * These numbers encoded in the optional property "riscv,hart-indexes"
+ * that should contain hart index for each interrupt destination in the same
+ * order as in the "interrupts-extended" property. If this property
+ * not exist, it assumed equal to the logical index, i.e. index within the
+ * "interrupts-extended" property.
+ *
+ * Return: error code
+ */
+int riscv_get_hart_index(struct fwnode_handle *fwnode, u32 logical_index,
+			 u32 *hart_index)
+{
+	static const char *prop_hart_index = "riscv,hart-indexes";
+	struct device_node *np = to_of_node(fwnode);
+
+	if (!np || !of_property_present(np, prop_hart_index)) {
+		*hart_index = logical_index;
+		return 0;
+	}
+
+	return of_property_read_u32_index(np, prop_hart_index,
+					  logical_index, hart_index);
+}
+
 #ifdef CONFIG_IRQ_STACKS
 #include <asm/irq_stack.h>