[PATCH] arm: irq: Allow for specification of no preallocated irqs

[Michael Bohan <mbohan@codeaurora.org>]

For cases with SPARSE_IRQ enabled, irqs preallocated with
arch_probe_nr_irqs() are already marked as allocated in the
allocated_irqs bitmap. As a consequence, irq chip drivers that
allocate irqs will feel one of two behaviors:

1. An allocation will succeed with the starting irq_base one
more than the preallocated irqs. This will thus waste the
preceeding interrupt resources that were preallocated, unless a
legacy chip driver happens to assume ownership of these by some
platform definition. The GIC driver is a typical primary chip
driver, and abides to the allocation APIs. So this can be a
problem in many trivial usecases.

2. An allocation will fail with < 0. This can also happen in the
GIC driver, which interprets this value as meaning the irq_descs
are already preallocated. But in Device Tree configurations, the
fallback irq_base is -1. This results in an invalid irq_base
value.

Looking forward, we are moving towards a world where preallocation
of irqs is no longer necessary. irq_domain is scoped to handle all
irq_desc allocations in the future. Thus, we should support
configurations where the platform wants to preallocate no irqs.

One easy way to achieve this is to allow for
machine_desc->nr_irqs < 0, which indicates not to preallocate any
interrupts.

Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
---
 arch/arm/include/asm/mach/arch.h |    2 +-
 arch/arm/kernel/irq.c            |   14 ++++++++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/arm/include/asm/mach/arch.h b/arch/arm/include/asm/mach/arch.h
index d7692ca..cc6506a 100644
--- a/arch/arm/include/asm/mach/arch.h
+++ b/arch/arm/include/asm/mach/arch.h
@@ -22,7 +22,7 @@ struct machine_desc {
 	const char *const 	*dt_compat;	/* array of device tree
 						 * 'compatible' strings	*/
 
-	unsigned int		nr_irqs;	/* number of IRQs */
+	int			nr_irqs;	/* number of IRQs */
 
 #ifdef CONFIG_ZONE_DMA
 	unsigned long		dma_zone_size;	/* size of DMA-able area */
diff --git a/arch/arm/kernel/irq.c b/arch/arm/kernel/irq.c
index 3efd82c..f74b173 100644
--- a/arch/arm/kernel/irq.c
+++ b/arch/arm/kernel/irq.c
@@ -129,8 +129,18 @@ void __init init_IRQ(void)
 #ifdef CONFIG_SPARSE_IRQ
 int __init arch_probe_nr_irqs(void)
 {
-	nr_irqs = machine_desc->nr_irqs ? machine_desc->nr_irqs : NR_IRQS;
-	return nr_irqs;
+	/*
+	 * machine_desc->nr_irqs < 0 is a special case that
+	 * specifies not to preallocate any irq_descs.
+	 */
+	if (machine_desc->nr_irqs < 0) {
+		nr_irqs = 0;
+		return nr_irqs;
+	} else {
+		nr_irqs = machine_desc->nr_irqs ?
+			  machine_desc->nr_irqs : NR_IRQS;
+		return nr_irqs;
+	}
 }
 #endif
 
-- 
1.7.8.3