[patch/rfc 1/2] GENIRQ: add handle_threaded_irq() flow handler

[David Brownell <david-b@pacbell.net>]

From: David Brownell <dbrownell@users.sourceforge.net>

Define a new flow handler, handle_threaded_irq(), for IRQ threads
to use when chaining IRQs.

Unlike existing flow handlers, handle_simple_irq() and siblings,
this one is used only from sleep-capable contexts.  It always calls
irqaction handlers from that same (shared) sleep-capable context.

This is independent of Thomas' irq threading patchset, and can be
viewed as a complement to it.  This adds support for IRQs whose
handlers must *ONLY* ever run in thread contexts ... instead of
offloading code from hardirq context into a thread.  Another way
this differs is that it doesn't create more kernel threads; it
only leverages an existing thread.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
---
 include/linux/irq.h |    7 ++++-
 kernel/irq/chip.c   |   61 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+), 2 deletions(-)

--- a/include/linux/irq.h
+++ b/include/linux/irq.h
@@ -283,8 +283,8 @@ static inline int irq_balancing_disabled
 extern int handle_IRQ_event(unsigned int irq, struct irqaction *action);
 
 /*
- * Built-in IRQ handlers for various IRQ types,
- * callable via desc->chip->handle_irq()
+ * IRQ flow handlers for various IRQ types, callable via
+ * generic_handle_irq*() or desc->handle_irq()
  */
 extern void handle_level_irq(unsigned int irq, struct irq_desc *desc);
 extern void handle_fasteoi_irq(unsigned int irq, struct irq_desc *desc);
@@ -293,6 +293,9 @@ extern void handle_simple_irq(unsigned i
 extern void handle_percpu_irq(unsigned int irq, struct irq_desc *desc);
 extern void handle_bad_irq(unsigned int irq, struct irq_desc *desc);
 
+/* Flow handler that must only be called from sleeping context */
+extern void handle_threaded_irq(unsigned int irq, struct irq_desc *desc);
+
 /*
  * Monolithic do_IRQ implementation.
  */
--- a/kernel/irq/chip.c
+++ b/kernel/irq/chip.c
@@ -295,6 +295,67 @@ static inline void mask_ack_irq(struct i
 }
 
 /**
+ *	handle_threaded_irq - flow handler reusing current irq thread
+ *	@irq:	the interrupt number
+ *	@desc:	the interrupt description structure for this irq
+ *	Context: irq thread, with IRQs enabled
+ *
+ *	IRQ threads which demultiplex IRQs may use this flow handler
+ *	to chain those demultiplexed IRQs to subsidiary handlers, when
+ *	all that IRQ dispatch logic must run in sleeping contexts.
+ *
+ *	Examples include some multifunction I2C and SPI based devices
+ *	(where access to registers, including ones involved in IRQ
+ *	dispatching, requires sleeping) that have multiple independent
+ *	maskable interupts.
+ *
+ *	The irq thread using this flow handler must handle any ack,
+ *	clear, mask or unmask issues needed.
+ */
+void
+handle_threaded_irq(unsigned int irq, struct irq_desc *desc)
+{
+	struct irqaction *action;
+	irqreturn_t action_ret;
+
+	spin_lock_irq(&desc->lock);
+
+	if (unlikely(desc->status & IRQ_INPROGRESS))
+		goto out_unlock;
+	desc->status &= ~(IRQ_REPLAY | IRQ_WAITING);
+	kstat_incr_irqs_this_cpu(irq, desc);
+
+	action = desc->action;
+	if (unlikely(!action || (desc->status & IRQ_DISABLED)))
+		goto out_unlock;
+
+	desc->status |= IRQ_INPROGRESS;
+	spin_unlock_irq(&desc->lock);
+
+	/* simplified handle_IRQ_event():  no random sampling;
+	 * IRQs are always enabled so action->handler may sleep;
+	 * no hooks for handing off to yet another irq thread.
+	 */
+	action_ret = IRQ_NONE;
+	do {
+		/* REVISIT can we get some explicit knowledge that this
+		 * handler expects to run in thread context?  Maybe an
+		 * IRQF_THREADED check, or a new handler type ...
+		 */
+		action_ret |= action->handler(irq, action->dev_id);
+		action = action->next;
+	} while (action);
+
+	if (!noirqdebug)
+		note_interrupt(irq, desc, action_ret);
+
+	spin_lock_irq(&desc->lock);
+	desc->status &= ~IRQ_INPROGRESS;
+out_unlock:
+	spin_unlock_irq(&desc->lock);
+}
+
+/**
  *	handle_simple_irq - Simple and software-decoded IRQs.
  *	@irq:	the interrupt number
  *	@desc:	the interrupt description structure for this irq