genirq: Add IRQF_NO_AUTOEN for request_irq/nmi()

Many drivers don't want interrupts enabled automatically via request_irq().
So they are handling this issue by either way of the below two:

(1)
  irq_set_status_flags(irq, IRQ_NOAUTOEN);
  request_irq(dev, irq...);

(2)
  request_irq(dev, irq...);
  disable_irq(irq);

The code in the second way is silly and unsafe. In the small time gap
between request_irq() and disable_irq(), interrupts can still come.

The code in the first way is safe though it's subobtimal.

Add a new IRQF_NO_AUTOEN flag which can be handed in by drivers to
request_irq() and request_nmi(). It prevents the automatic enabling of the
requested interrupt/nmi in the same safe way as Rust-for-Linux#1 above. With that the
various usage sites of Rust-for-Linux#1 and Rust-for-Linux#2 above can be simplified and corrected.

Signed-off-by: Barry Song <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]

Author: Barry Song

include/linux/interrupt.h

@@ -61,6 +61,9 @@
  *                interrupt handler after suspending interrupts. For system
  *                wakeup devices users need to implement wakeup detection in
  *                their interrupt handlers.
+ * IRQF_NO_AUTOEN - Don't enable IRQ or NMI automatically when users request it.
+ *                Users will enable it explicitly by enable_irq() or enable_nmi()
+ *                later.
  */
 #define IRQF_SHARED		0x00000080
 #define IRQF_PROBE_SHARED	0x00000100
@@ -74,6 +77,7 @@
 #define IRQF_NO_THREAD		0x00010000
 #define IRQF_EARLY_RESUME	0x00020000
 #define IRQF_COND_SUSPEND	0x00040000
+#define IRQF_NO_AUTOEN		0x00080000
 
 #define IRQF_TIMER		(__IRQF_TIMER | IRQF_NO_SUSPEND | IRQF_NO_THREAD)
 

kernel/irq/manage.c

@@ -1693,7 +1693,8 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 			irqd_set(&desc->irq_data, IRQD_NO_BALANCING);
 		}
 
-		if (irq_settings_can_autoenable(desc)) {
+		if (!(new->flags & IRQF_NO_AUTOEN) &&
+		    irq_settings_can_autoenable(desc)) {
 			irq_startup(desc, IRQ_RESEND, IRQ_START_COND);
 		} else {
 			/*
@@ -2086,10 +2087,15 @@ int request_threaded_irq(unsigned int irq, irq_handler_t handler,
 	 * which interrupt is which (messes up the interrupt freeing
 	 * logic etc).
 	 *
+	 * Also shared interrupts do not go well with disabling auto enable.
+	 * The sharing interrupt might request it while it's still disabled
+	 * and then wait for interrupts forever.
+	 *
 	 * Also IRQF_COND_SUSPEND only makes sense for shared interrupts and
 	 * it cannot be set along with IRQF_NO_SUSPEND.
 	 */
 	if (((irqflags & IRQF_SHARED) && !dev_id) ||
+	    ((irqflags & IRQF_SHARED) && (irqflags & IRQF_NO_AUTOEN)) ||
 	    (!(irqflags & IRQF_SHARED) && (irqflags & IRQF_COND_SUSPEND)) ||
 	    ((irqflags & IRQF_NO_SUSPEND) && (irqflags & IRQF_COND_SUSPEND)))
 		return -EINVAL;
@@ -2245,7 +2251,8 @@ int request_nmi(unsigned int irq, irq_handler_t handler,
 
 	desc = irq_to_desc(irq);
 
-	if (!desc || irq_settings_can_autoenable(desc) ||
+	if (!desc || (irq_settings_can_autoenable(desc) &&
+	    !(irqflags & IRQF_NO_AUTOEN)) ||
 	    !irq_settings_can_request(desc) ||
 	    WARN_ON(irq_settings_is_per_cpu_devid(desc)) ||
 	    !irq_supports_nmi(desc))