linux-kernel - Re: [PATCH v2 1/2] ARM: davinci: Allocate extra interrupts

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Date:   Wed, 18 Jan 2017 10:57:31 -0600
From:   David Lechner <david@...hnology.com>
To:     Sekhar Nori <nsekhar@...com>, linux-arm-kernel@...ts.infradead.org
Cc:     Kevin Hilman <khilman@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] ARM: davinci: Allocate extra interrupts

On 01/18/2017 03:50 AM, Sekhar Nori wrote:
> On Saturday 14 January 2017 01:30 AM, David Lechner wrote:
>> This allocates extra interrupts for mach-davinci. These extra interrupts
>> are need for things like IIO triggers.
>
> I am not really familiar with IIO triggers. Can you give some more
> detail on what fails without this patch?

A trigger is used to initiate the reading of an iio device. For example, 
there is a mechanism for a sysfs trigger. When you write 1 to the sysfs 
attribute, it triggers an interrupt that is handled by the iio device.

Since these triggers use interrupts, you need to allocate spare 
interrupts in order to set up the trigger. Otherwise, setting up the 
trigger will fail with an error code (I forgot which one exactly) 
because all of the allocated irqs have already been assigned to hardware 
irqs and are not available.

Here is where the iio subsytem actually allocates the irq:
http://lxr.free-electrons.com/source/drivers/iio/industrialio-trigger.c#L525