From mboxrd@z Thu Jan  1 00:00:00 1970
From: dhylands@gmail.com (Dave Hylands)
Date: Wed, 2 Mar 2011 15:49:58 -0700
Subject: Disabling an interrupt
In-Reply-To: <4D6CB46C.4040701@gmail.com>
References: <4D6CB46C.4040701@gmail.com>
Message-ID: <AANLkTimeKux1BSwucqmW12moF7zeH9RFeSCeGHPRpVkx@mail.gmail.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

Hi Jacky,

Sending to the list as well.

On Tue, Mar 1, 2011 at 1:55 AM, Jacky Lam <lamshuyin@gmail.com> wrote:
> Hi,
>
>     It's long before when I want to enable/disable an interrupt, I call
> enable_irq()/disable_irq(). However, recently, I do that again.
> disable_irq() do nothing. I looked into the code and find disable_irq()
> is pointing to a empty function default_disable(). This change is
> started from 2.6.20.
>
>     I want to know what should I do if I want to disable an interrupt now?

So disable/enable_irq are nestable, and you're expected to call them
in the order disable/enable.

You need to call enable_irq exactly the same number of times that you
call disable_irq.

If you start wth inerrtupts enabled and do
enable_irq
...do some stuff...
disable_irq

then disable_irq will do nothing since it just decremented the count
that enable_irq incremented.

Another way of looking at it is that disable_irq increments a count,
and enable_irq decrements a count.
The interrupt is only "really" disabled when the count transitions
from 0 to 1, and the interrupt is only "really" enabled when the count
transitions from 1 to 0.

Dave Hylands