Disabling an interrupt

Disabling an interrupt

[Jacky Lam]

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?

     Thanks.

Jacky

Disabling an interrupt

[Sri Ram Vemulpali]

http://lxr.linux.no/linux+v2.6.37.2/kernel/irq/manage.c#L217

Check the above link.

--Sri

On Tue, Mar 1, 2011 at 3:55 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?
>
> ? ? Thanks.
>
> Jacky
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>



-- 
Regards,
Sri.

Disabling an interrupt

[Dave Hylands]

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

Disabling an interrupt

[Jacky Lam]

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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Disabling an interrupt

[Dave Hylands]

Hi Jacky,

On Wed, Mar 2, 2011 at 6:26 PM, Jacky Lam <lamshuyin@gmail.com> wrote:
> Hi Dave,
>
> ??? I have confirmed the execution has go into
> ??? ??? desc->irq_data.chip->irq_disable(&desc->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.

Which irq chip is being used?

It's really up to the individual irq chip driver to implement things
in an appropriate manner.

Since the IRQ_DISABLED flag is set, if the interrupt happens to fire
again, your handler shouldn't be called. The generic handler masks
interrupts when they occur, and if the IRQ_DISABLED flag is set then
it will leave the interrupt masked (i.e. disabled). Look at the the
__do_IRQ function in handle.c
<http://lxr.linux.no/linux+v2.6.36.4/kernel/irq/handle.c#L446>

So even though it looks like its not doing things, the interrupt is
still effectively disabled.

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com