genirq: Allow irq_chip registration functions to take a const irq_chip

In order to let a const irqchip be fed to the irqchip layer, adjust
the various prototypes. An extra cast in irq_set_chip()() is required
to avoid a warning.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220209162607.1118325-3-maz@kernel.org

Author: Marc Zyngier

include/linux/irq.h

@@ -710,10 +710,11 @@ extern struct irq_chip no_irq_chip;
 extern struct irq_chip dummy_irq_chip;
 
 extern void
-irq_set_chip_and_handler_name(unsigned int irq, struct irq_chip *chip,
+irq_set_chip_and_handler_name(unsigned int irq, const struct irq_chip *chip,
 			      irq_flow_handler_t handle, const char *name);
 
-static inline void irq_set_chip_and_handler(unsigned int irq, struct irq_chip *chip,
+static inline void irq_set_chip_and_handler(unsigned int irq,
+					    const struct irq_chip *chip,
 					    irq_flow_handler_t handle)
 {
 	irq_set_chip_and_handler_name(irq, chip, handle, NULL);
@@ -803,7 +804,7 @@ static inline void irq_set_percpu_devid_flags(unsigned int irq)
 }
 
 /* Set/get chip/data for an IRQ: */
-extern int irq_set_chip(unsigned int irq, struct irq_chip *chip);
+extern int irq_set_chip(unsigned int irq, const struct irq_chip *chip);
 extern int irq_set_handler_data(unsigned int irq, void *data);
 extern int irq_set_chip_data(unsigned int irq, void *data);
 extern int irq_set_irq_type(unsigned int irq, unsigned int type);

kernel/irq/chip.c

@@ -38,18 +38,15 @@ struct irqaction chained_action = {
  *	@irq:	irq number
  *	@chip:	pointer to irq chip description structure
  */
-int irq_set_chip(unsigned int irq, struct irq_chip *chip)
+int irq_set_chip(unsigned int irq, const struct irq_chip *chip)
 {
 	unsigned long flags;
 	struct irq_desc *desc = irq_get_desc_lock(irq, &flags, 0);
 
 	if (!desc)
 		return -EINVAL;
 
-	if (!chip)
-		chip = &no_irq_chip;
-
-	desc->irq_data.chip = chip;
+	desc->irq_data.chip = (struct irq_chip *)(chip ?: &no_irq_chip);
 	irq_put_desc_unlock(desc, flags);
 	/*
 	 * For !CONFIG_SPARSE_IRQ make the irq show up in
@@ -1073,7 +1070,7 @@ irq_set_chained_handler_and_data(unsigned int irq, irq_flow_handler_t handle,
 EXPORT_SYMBOL_GPL(irq_set_chained_handler_and_data);
 
 void
-irq_set_chip_and_handler_name(unsigned int irq, struct irq_chip *chip,
+irq_set_chip_and_handler_name(unsigned int irq, const struct irq_chip *chip,
 			      irq_flow_handler_t handle, const char *name)
 {
 	irq_set_chip(irq, chip);