genirq/debugfs: Triggering of interrupts from userspace

When developing new (and therefore buggy) interrupt related
code, it can sometimes be useful to inject interrupts without
having to rely on a device to actually generate them.

This functionnality relies either on the irqchip driver to
expose a irq_set_irqchip_state(IRQCHIP_STATE_PENDING) callback,
or on the core code to be able to retrigger a (edge-only)
interrupt.

To use this feature:

echo -n trigger > /sys/kernel/debug/irq/irqs/IRQNUM

WARNING: This is DANGEROUS, and strictly a debug feature.
Do not use it on a production system. Your HW is likely to
catch fire, your data to be corrupted, and reporting this will
make you look an even bigger fool than the idiot who wrote
this patch.
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170818081156.9264-1-marc.zyngier@arm.com

kernel/irq/debugfs.c

@@ -5,6 +5,7 @@
  */
 #include <linux/irqdomain.h>
 #include <linux/irq.h>
+#include <linux/uaccess.h>
 
 #include "internals.h"
 
@@ -171,8 +172,55 @@ static int irq_debug_open(struct inode *inode, struct file *file)
 	return single_open(file, irq_debug_show, inode->i_private);
 }
 
+static ssize_t irq_debug_write(struct file *file, const char __user *user_buf,
+			       size_t count, loff_t *ppos)
+{
+	struct irq_desc *desc = file_inode(file)->i_private;
+	char buf[8] = { 0, };
+	size_t size;
+
+	size = min(sizeof(buf) - 1, count);
+	if (copy_from_user(buf, user_buf, size))
+		return -EFAULT;
+
+	if (!strncmp(buf, "trigger", size)) {
+		unsigned long flags;
+		int err;
+
+		/* Try the HW interface first */
+		err = irq_set_irqchip_state(irq_desc_get_irq(desc),
+					    IRQCHIP_STATE_PENDING, true);
+		if (!err)
+			return count;
+
+		/*
+		 * Otherwise, try to inject via the resend interface,
+		 * which may or may not succeed.
+		 */
+		chip_bus_lock(desc);
+		raw_spin_lock_irqsave(&desc->lock, flags);
+
+		if (irq_settings_is_level(desc)) {
+			/* Can't do level, sorry */
+			err = -EINVAL;
+		} else {
+			desc->istate |= IRQS_PENDING;
+			check_irq_resend(desc);
+			err = 0;
+		}
+
+		raw_spin_unlock_irqrestore(&desc->lock, flags);
+		chip_bus_sync_unlock(desc);
+
+		return err ? err : count;
+	}
+
+	return count;
+}
+
 static const struct file_operations dfs_irq_ops = {
 	.open		= irq_debug_open,
+	.write		= irq_debug_write,
 	.read		= seq_read,
 	.llseek		= seq_lseek,
 	.release	= single_release,
@@ -186,7 +234,7 @@ void irq_add_debugfs_entry(unsigned int irq, struct irq_desc *desc)
 		return;
 
 	sprintf(name, "%d", irq);
-	desc->debugfs_file = debugfs_create_file(name, 0444, irq_dir, desc,
+	desc->debugfs_file = debugfs_create_file(name, 0644, irq_dir, desc,
 						 &dfs_irq_ops);
 }