irqchip/riscv-imsic: Add a CPU pm notifier to restore the IMSIC on exit

The IMSIC might be reset when the system enters a low power state, but on
exit nothing restores the registers, which prevents interrupt delivery.

Solve this by registering a CPU power management notifier, which restores
the IMSIC on exit.

Signed-off-by: Nick Hu <nick.hu@sifive.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Cyan Yang <cyan.yang@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Nutty Liu <liujingqi@lanxincomputing.com>
Link: https://patch.msgid.link/20251202-preserve-aplic-imsic-v3-1-1844fbf1fe92@sifive.com

Author: nick650823

drivers/irqchip/irq-riscv-imsic-early.c

@@ -7,6 +7,7 @@
 #define pr_fmt(fmt) "riscv-imsic: " fmt
 #include <linux/acpi.h>
 #include <linux/cpu.h>
+#include <linux/cpu_pm.h>
 #include <linux/export.h>
 #include <linux/interrupt.h>
 #include <linux/init.h>
@@ -123,14 +124,8 @@ static void imsic_handle_irq(struct irq_desc *desc)
 	chained_irq_exit(chip, desc);
 }
 
-static int imsic_starting_cpu(unsigned int cpu)
+static void imsic_hw_states_init(void)
 {
-	/* Mark per-CPU IMSIC state as online */
-	imsic_state_online();
-
-	/* Enable per-CPU parent interrupt */
-	enable_percpu_irq(imsic_parent_irq, irq_get_trigger_type(imsic_parent_irq));
-
 	/* Setup IPIs */
 	imsic_ipi_starting_cpu();
 
@@ -142,6 +137,18 @@ static int imsic_starting_cpu(unsigned int cpu)
 
 	/* Enable local interrupt delivery */
 	imsic_local_delivery(true);
+}
+
+static int imsic_starting_cpu(unsigned int cpu)
+{
+	/* Mark per-CPU IMSIC state as online */
+	imsic_state_online();
+
+	/* Enable per-CPU parent interrupt */
+	enable_percpu_irq(imsic_parent_irq, irq_get_trigger_type(imsic_parent_irq));
+
+	/* Initialize the IMSIC registers to enable the interrupt delivery */
+	imsic_hw_states_init();
 
 	return 0;
 }
@@ -157,6 +164,22 @@ static int imsic_dying_cpu(unsigned int cpu)
 	return 0;
 }
 
+static int imsic_pm_notifier(struct notifier_block *self, unsigned long cmd, void *v)
+{
+	switch (cmd) {
+	case CPU_PM_EXIT:
+		/* Initialize the IMSIC registers to enable the interrupt delivery */
+		imsic_hw_states_init();
+		break;
+	}
+
+	return NOTIFY_OK;
+}
+
+static struct notifier_block imsic_pm_notifier_block = {
+	.notifier_call = imsic_pm_notifier,
+};
+
 static int __init imsic_early_probe(struct fwnode_handle *fwnode)
 {
 	struct irq_domain *domain;
@@ -194,7 +217,7 @@ static int __init imsic_early_probe(struct fwnode_handle *fwnode)
 	cpuhp_setup_state(CPUHP_AP_IRQ_RISCV_IMSIC_STARTING, "irqchip/riscv/imsic:starting",
 			  imsic_starting_cpu, imsic_dying_cpu);
 
-	return 0;
+	return cpu_pm_register_notifier(&imsic_pm_notifier_block);
 }
 
 static int __init imsic_early_dt_init(struct device_node *node, struct device_node *parent)