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Date: Wed, 16 Jun 2010 20:19:41 +0200
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
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Subject: Re: [Xenomai-help] How to avoid a tight real time loop
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To: Alexis Berlemont <alexis.berlemont@domain.hid>
Cc: xenomai@xenomai.org

Alexis Berlemont wrote:
> Hi,
> 
> Daniele Nicolodi wrote:
>> Hello. In my application I need to read data from an ADC card in blocks
>> of a given number of samples. For efficiency I'm using an mmapped buffer
>> to exchange data with the ADC driver. I'm setting up the acquisition,
>> and then calling a loop like this:
>>
>> unsigned int required = <compute required number of bytes>;
>> unsigned int read = 0;
>> while (1) {
>>   while (read < required) {
>>     read = a4l_pool(...);
>>   }
>>   process(buffer, ...);
>> }
>>
>> However if I run this loop in a real time thread the tight while loop is
>> an obvious cpu hog: the watchdog kicks in and kills the thread.
>>
>> There is not an analogy API that can be used to require a certain amount
>> of data to the ADC driver. How can I code this loop to let other
>> processes to run?
> 
> As a matter of fact, there is something in the API which could have
> helped you in an unmapped configuration. If you set the flag
> A4L_CMD_BULK in the field "flags" of the command structure, a call to
> a4l_async_read() only returns when the data size specified as argument
> is available. 
> 
> Unfortunately, I have not done the same thing with a4l_poll, I might
> have added the requested size as an additional argument to a4l_poll. 
> 
> Any idea on how to extend the API without breaking it ?

Maybe it would be possible to use the standard "select" call, with a
threshold configurable with an ioctl ?


-- 
					    Gilles.