Re: [Xenomai-help] parallelport module for measuring external interrupt latency

[Jan Kiszka <jan.kiszka@domain.hid>]

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Markus Franke wrote:
> Jan Kiszka wrote:
>>> // ISR
>>> int parport_isr(xnintr_t* cookie)
>>> {
>>> 	rdtsc(t_end);
>>> 	
>>> 	outb(0x00,SPPDATAPORT);
>>> 	
>>> #ifdef DEBUG
>>> 	printk(KERN_INFO "parport_latency: Interrupt fired!!!\n");
>>> 	printk(KERN_INFO "parport_latency: interruptcount before = %d!!!\n",atomic_read(&interruptcount));
>>> #endif
>>> 	
>>> 	atomic_inc(&interruptcount);
>>>
>>> #ifdef DEBUG
>>> 	printk(KERN_INFO "parport_latency: interruptcount after= %d!!!\n",atomic_read(&interruptcount));
>>> #endif
>>>
>>> 	wake_up_interruptible(&intlatpar_queue);
>>
>> This is a hard-RT IRQ handler, thus any scheduling Linux service is
>> strictly forbidden.
>>
>> [Reminds me of the I-pipe debugging service that can catch such faults
>> but still needs some integration work...]
> 
> Ok I understand. But somehow I have to notify the read()-call that it
> can compute the latency value. Do you have any suggestions how to do that?

Even if the notification is not time-critical, you first have to get out
of the Xenomai context in a sane manner. rtdm_nrtsig may help if you
don't want to use the RT scheduler for user task wakeup.

> 
>>> 	if(register_chrdev(INTLAT_DEV_MAJOR,INTLAT_DEV_NAME,&intlat_fops) < 0) 
>>> 	{
>>> 		printk(KERN_WARNING "intlat: parport_latency(): register_chrdev() failed.\n");
>>> 		rt_intr_disable(&intr);
>>> 		return -1;
>>> 	}
>>
>> Hmm, a Linux character device for this purpose... I guess you should
>> study the existing irqbench test a bit first. And if you don't see why
>> something is done the way it is - ask here.
> 
> Yes of course I have already studied the irqbench test but it uses the
> RTDM skin. I also know that this skin should be used for developing hard
> realtime drivers. But I just need to measure latencies and I don't want
> to develop a huge realtime driver. I just need some values for

Well, irqbench contains a bunch of test cases. If you cut it down to
only your scenario, it wouldn't appear as that "huge", it would likely
be as long as your own driver.

> comparison with the driver working on plain linux. I was wondering
> whether it is possible to rewrite the driver working on plain linux by
> using the Xenomai native API.
> Why isn't it possible to use a Linux character device. I just want to
> transfer the latency values to the user space task which is out of any
> realtime context I think.

You can - if you keep a clean separation between RT and non-RT. Direct
wakeup from the Xenomai IRQ handler is forbidden, but indirect via a
rtdm_nrtsig trampoline would be feasible. [In that case you could also
drop all Xenomai functions from your user-space tool and use plain Linux.]

Jan


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