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    SubjectRe: [PATCH] arm: irq: Allow for specification of no preallocated irqs
    On 01/19/2012 04:43 PM, Michael Bohan wrote:
    > 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>

    I don't know if you saw my recent series on NR_IRQS clean-up (Make
    mach/irqs.h optional).

    No doubt that arch_probe_nr_irqs is doing the wrong thing on ARM, but no
    pre-allocation is not what we want either. We ultimately want
    arch_probe_nr_irqs to return NR_IRQS_LEGACY (16) to reserve IRQ0 (aka
    NO_IRQ) and legacy ISA IRQs. With my series, NR_IRQS is set to
    NR_IRQS_LEGACY for SPARSE_IRQ. You can accomplish the same thing without
    that series by setting .nr_irqs to NR_IRQS for non-DT and to
    NR_IRQS_LEGACY for DT. For platforms to work in single kernel builds,
    they will need to select SPARSE_IRQ.

    Rob

    > ---
    > 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
    >



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