From mboxrd@z Thu Jan  1 00:00:00 1970
From: lamshuyin@gmail.com (Jacky Lam)
Date: Thu, 03 Mar 2011 09:26:26 +0800
Subject: Disabling an interrupt
In-Reply-To: <AANLkTimeKux1BSwucqmW12moF7zeH9RFeSCeGHPRpVkx@mail.gmail.com>
References: <4D6CB46C.4040701@gmail.com>
	<AANLkTimeKux1BSwucqmW12moF7zeH9RFeSCeGHPRpVkx@mail.gmail.com>
Message-ID: <4D6EEE42.10909@gmail.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

Hi Dave,

     I have confirmed the execution has go into
desc <http://lxr.linux.no/linux+*/+code=desc>->irq_data 
<http://lxr.linux.no/linux+*/+code=irq_data>.chip 
<http://lxr.linux.no/linux+*/+code=chip>->irq_disable 
<http://lxr.linux.no/linux+*/+code=irq_disable>(&desc 
<http://lxr.linux.no/linux+*/+code=desc>->irq_data 
<http://lxr.linux.no/linux+*/+code=irq_data>);

   However, irq_disable points to kernel/irq/chip.c:default_disable() which do nothings.
   Unlike default_enable() which called by enable_irq() will unmask the IRQ accordingly.

   I don't understand the reason behind.

BR,
Jacky
	

On 3/3/2011 6:49 AM, Dave Hylands wrote:
> 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
>

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