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SubjectRe: [patch 3/3] genirq: introduce IRQF_ALLOW_NESTED flag for request_irq()
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On Thu, 11 Mar 2010, akpm@linux-foundation.org wrote:

Sorry, this dropped from my radar.

> From: Marc Zyngier <maz@misterjones.org>
>
> Now that we enjoy threaded interrupts, we're starting to see irq_chip
> implementations (wm831x, pca953x) that make use of threaded interrupts for
> the controller, and nested interrupts for the client interrupt. It all
> works very well, with one drawback:
>
> Drivers requesting an IRQ must now know whether the handler will run in a
> thread context of not, and call request_threaded_irq() or request_irq()
> accordingly.
>
> The problem is that the requesting driver sometimes doesn't know about the
> nature of the interrupt, specially when the interrupt controller is a
> discrete chip (typically a GPIO expander connected over I2C) that can be
> connected to a wide variety of otherwise perfectly supported hardware.
>
> The following patch introduces the IRQF_ALLOW_NESTED flag, that acts as a
> "contract" between the driver and the genirq framework. By using this
> flag as part of the request_irq() call, the driver informs the genirq
> framework that the handler will happily run either in hardirq or nested
> context, without any ill effect.
>
> The benefit of this is a single API for drivers. It still requires the
> driver to be audited, and the flag added to the request_irq() call.
>
> Of course, this goes against Linus choice of having separate APIs for both
> cases. The only alternative I can imagine for this is to use
> request_threaded_irq() with the same function provided for both the
> handler and the threaded handler. Ugly...

In general I have no objections, but one thing bothers me. We have no
way to let a driver know whether it runs in a nested threaded context
or in hard irq context. There might be (future) drivers which would be
happy to know that to apply context dependent optimizations etc.

What about a new function which solves your problem and returns that
information ? Something along the line:

int request_any_context_irq(....)
{
...
if (desc->status & IRQ_NESTED_THREAD) {
ret = request_threaded_irq();
if (!ret)
ret = IRQ_IS_NESTED;
} else {
.....
ret = IRQ_IS_NONTHREADED;
else
ret = IRQ_IS_THREADED;
}
...
return ret;
}
You get the idea, right ?

It's a bit more code, but less magic and more flexible for further use
cases.

Thanks,

tglx

> This patch has been tested on ARM and Blackfin platforms.
>
> Signed-off-by: Marc Zyngier <maz@misterjones.org>
> Tested-by: Joachim Eastwood <joachim.eastwood@jotron.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
> include/linux/interrupt.h | 3 +++
> kernel/irq/manage.c | 12 +++++++++---
> 2 files changed, 12 insertions(+), 3 deletions(-)
>
> diff -puN include/linux/interrupt.h~genirq-introduce-irqf_allow_nested-flag-for-request_irq include/linux/interrupt.h
> --- a/include/linux/interrupt.h~genirq-introduce-irqf_allow_nested-flag-for-request_irq
> +++ a/include/linux/interrupt.h
> @@ -52,6 +52,8 @@
> * IRQF_ONESHOT - Interrupt is not reenabled after the hardirq handler finished.
> * Used by threaded interrupts which need to keep the
> * irq line disabled until the threaded handler has been run.
> + * IRQF_ALLOW_NESTED - Handler can be either run as hardirq or nested
> + * interrupt.
> */
> #define IRQF_DISABLED 0x00000020
> #define IRQF_SAMPLE_RANDOM 0x00000040
> @@ -62,6 +64,7 @@
> #define IRQF_NOBALANCING 0x00000800
> #define IRQF_IRQPOLL 0x00001000
> #define IRQF_ONESHOT 0x00002000
> +#define IRQF_ALLOW_NESTED 0x00004000
>
> /*
> * Bits used by threaded handlers:
> diff -puN kernel/irq/manage.c~genirq-introduce-irqf_allow_nested-flag-for-request_irq kernel/irq/manage.c
> --- a/kernel/irq/manage.c~genirq-introduce-irqf_allow_nested-flag-for-request_irq
> +++ a/kernel/irq/manage.c
> @@ -641,12 +641,18 @@ __setup_irq(unsigned int irq, struct irq
>
> /*
> * Check whether the interrupt nests into another interrupt
> - * thread.
> + * thread. Nested interrupt must provide a thread function
> + * unless it raises the IRQF_ALLOW_NESTED flag.
> */
> nested = desc->status & IRQ_NESTED_THREAD;
> if (nested) {
> - if (!new->thread_fn)
> - return -EINVAL;
> + if (!new->thread_fn) {
> + if (new->flags & IRQF_ALLOW_NESTED)
> + new->thread_fn = new->handler;
> + else
> + return -EINVAL;
> + }
> +
> /*
> * Replace the primary handler which was provided from
> * the driver for non nested interrupt handling by the
> _
>


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