Triggering UserCode from KernelModule (IRQ)

Triggering UserCode from KernelModule (IRQ)

[M. Koehrer]

Hi all!

I am currently evaluting the RT_PREEMPT kernel as a replacement for a Xenomai based real time application.
(Intel Core2Duo PCs).
So far, everything works really fine and the performance is about the same as with Xenomai.
Great work!

However, I have one question:
I have a PCI hardware I/O board that exchanges data with the user mode 
directly (by using mmap), this works fine.
However, there is one operation mode that uses interrupts to signal incoming data.
The interrupt handler is realized as a tiny kernel module (that one that provides also 
the mmap for the user mode application).
Now I am not sure how the signalling of the IRQ from kernel to user mode could be realized 
best (in terms of low latency).

The "classical" approach for this task is to provide a character driver with a .poll file operation.
In Xenomai I could use a semaphore that was accessible from kernel and user space for this.
What is the best way in the RT_PREEMPT "world"? 

Thanks a lot for all feedback on this question.

Regards

Mathias


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Re: Triggering UserCode from KernelModule (IRQ)

[Daniel Walker]

On Wed, 2009-04-22 at 16:14 +0200, M. Koehrer wrote:
> Hi all!
> 
> I am currently evaluting the RT_PREEMPT kernel as a replacement for a Xenomai based real time application.
> (Intel Core2Duo PCs).
> So far, everything works really fine and the performance is about the same as with Xenomai.
> Great work!

I thought I saw something someplace about Xenomai supporting preempt_rt
as a real time layer. So you might not need to do much to your
application in order to switch ..

Daniel

Re: Triggering UserCode from KernelModule (IRQ)

[Robert Schwebel]

On Wed, Apr 22, 2009 at 04:14:48PM +0200, M. Koehrer wrote:
> However, there is one operation mode that uses interrupts to signal
> incoming data.  The interrupt handler is realized as a tiny kernel
> module (that one that provides also the mmap for the user mode
> application).  Now I am not sure how the signalling of the IRQ from
> kernel to user mode could be realized best (in terms of low latency).
>
> The "classical" approach for this task is to provide a character
> driver with a .poll file operation. In Xenomai I could use a semaphore
> that was accessible from kernel and user space for this. What is the
> best way in the RT_PREEMPT "world"?

UIO might be a good choice here, the rest is giving the interrupt
handler and the processes the right priorities.

rsc
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Re: Re: Triggering UserCode from KernelModule (IRQ)

[M. Koehrer]

Hi Claus, 

thanks for the feedback.
> simply write an UIO driver for your I/O-HW. You find an excellent howto in
> http://www.osadl.org/UIO.uio0.0.html
> 
> The UIO-driver has 2 benefits for you:
> 1. It provides you with direct access to the mmapped area from user space, 
> freeing you from the necessity to communicate via the /dev interface.
> 2. It can invoke a handler function in user space, if an interrupt occurs.
That is really cool for a new driver, however I have an already existing driver
(that works with Xenomai) and in the first step I do not want to change too much...
The only missing piece here is the "triggering" of my user space task whenever
an interrupt occurred.
Is the .poll approach real time capable or is there another approach better?

Thanks a lot

Regards

Mathias

-- 
Mathias Koehrer
mathias_koehrer@arcor.de


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