linux-kernel - Re: [PATCH] gpio: zynq: Implement irq_(request|release)

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Date:	Tue, 27 Oct 2015 17:37:29 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Linus Walleij <linus.walleij@...aro.org>,
	Soren Brinkmann <soren.brinkmann@...inx.com>
CC:	Alexandre Courbot <gnurou@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	Michal Simek <michal.simek@...inx.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	John Linn <linnj@...inx.com>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] gpio: zynq: Implement irq_(request|release)_resources

On 10/27/2015 04:53 PM, Linus Walleij wrote:
> On Fri, Oct 23, 2015 at 3:36 PM, Soren Brinkmann
> <soren.brinkmann@...inx.com> wrote:
> 
>> The driver uses runtime PM to leverage low power techniques. For
>> use-cases using GPIO as interrupt the device needs to be in an
>> appropriate state.
>>
>> Reported-by: John Linn <linnj@...inx.com>
>> Signed-off-by: Soren Brinkmann <soren.brinkmann@...inx.com>
>> Tested-by: John Linn <linnj@...inx.com>
> 
> As pointed out by Grygorii in
> commit aca82d1cbb49af34b69ecd4571a0fe48ad9247c1:
> 
>     The PM runtime API can't be used in atomic contex on -RT even if
>     it's configured as irqsafe. As result, below error report can
>     be seen when PM runtime API called from IRQ chip's callbacks
>     irq_startup/irq_shutdown/irq_set_type, because they are
>     protected by RAW spinlock:
> (...)
>     The IRQ chip interface defines only two callbacks which are executed in
>     non-atomic contex - irq_bus_lock/irq_bus_sync_unlock, so lets move
>     PM runtime calls there.
> 
> I.e. these calls are atomic context and it's just luck that it works
> and this is fragile.
> 
> Can you please check if you can move it to
> irq_bus_lock()/irq_sync_unlock()
> like Grygorii does?

That only powers up the chip when the chip is accessed. For proper IRQ
operation the chip needs to be powered up though as long as the IRQ is
enabled. request_irq() and free_irq() must always be called from sleepable
context. The thing is just that request_resource/release_resource are called
from within a raw spinlock, which is necessary since otherwise you can't
guarantee that they are only called once for shared interrupts.

It might make sense to add a separate set of callbacks to the irq_chip
struct that are called from the sleepable sections of
request_irq()/free_irq() which are meant for power management purposes and
which wont have the guarantee that they are only called once for shared IRQs
(but are still balanced).

Thomas, do you have any thoughts on this?

- Lars

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