genirq: Expose interrupt information through sysfs

Information about interrupts is exposed via /proc/interrupts, but the
format of that file has changed over kernel versions and differs across
architectures. It also has varying column numbers depending on hardware.

That all makes it hard for tools to parse.

To solve this, expose the information through sysfs so each irq attribute
is in a separate file in a consistent, machine parsable way.

This feature is only available when both CONFIG_SPARSE_IRQ and
CONFIG_SYSFS are enabled.

Examples:
  /sys/kernel/irq/18/actions:	i801_smbus,ehci_hcd:usb1,uhci_hcd:usb7
  /sys/kernel/irq/18/chip_name:	IR-IO-APIC
  /sys/kernel/irq/18/hwirq:		18
  /sys/kernel/irq/18/name:		fasteoi
  /sys/kernel/irq/18/per_cpu_count:	0,0
  /sys/kernel/irq/18/type:		level

  /sys/kernel/irq/25/actions:	ahci0
  /sys/kernel/irq/25/chip_name:	IR-PCI-MSI
  /sys/kernel/irq/25/hwirq:		512000
  /sys/kernel/irq/25/name:		edge
  /sys/kernel/irq/25/per_cpu_count:	29036,0
  /sys/kernel/irq/25/type:		edge

[ tglx: Moved kobject_del() under sparse_irq_lock, massaged code comments
  	and changelog ]

Signed-off-by: Craig Gallek <kraig@google.com>
Cc: David Decotigny <decot@google.com>
Link: http://lkml.kernel.org/r/1473783291-122873-1-git-send-email-kraigatgoog@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/irqdesc.h

@@ -2,6 +2,7 @@
 #define _LINUX_IRQDESC_H
 
 #include <linux/rcupdate.h>
+#include <linux/kobject.h>
 
 /*
  * Core internal functions to deal with irq descriptors
@@ -43,6 +44,7 @@ struct pt_regs;
  * @force_resume_depth:	number of irqactions on a irq descriptor with
  *			IRQF_FORCE_RESUME set
  * @rcu:		rcu head for delayed free
+ * @kobj:		kobject used to represent this struct in sysfs
  * @dir:		/proc/irq/ procfs entry
  * @name:		flow handler name for /proc/interrupts output
  */
@@ -88,6 +90,7 @@ struct irq_desc {
 #endif
 #ifdef CONFIG_SPARSE_IRQ
 	struct rcu_head		rcu;
+	struct kobject		kobj;
 #endif
 	int			parent_irq;
 	struct module		*owner;