spi_async and heavy CPU occupation by ksoftirqd

[llandre <r&d2-4VKA1VU3ct/j+vYz1yj4TQ@public.gmane.org>]

I'm facing the following problem.
I have to poll a SPI device periodically (around ten times a second). 
Both command and response are 1 byte. To do the job I created a work 
that looks like this:

work(...)
{
	sets debug_pin
	poll the device
	err = queue_delayed_work(wq, work, (HZ / 10));
	clears debug_pin
}

The actual SPI transaction is perfomed by invoking spi_async. SPI 
transactions work fine but the kernel daemon ksoftirqd eats about 18% of 
CPU time. With the help of debug pin, I measured the execution time of 
function work that is around 70us. Then I commented out the call of 
spi_async and the ksoftirqd CPU occupation drops to 0%. Am I doing 
something wrong?

I'm working with kernel 2.6.21.3 on iMXL ARM9 processor.

TIA,
llandre

DAVE Electronics System House - R&D Department
web:   http://www.dave-tech.it
email: r&d2-4VKA1VU3ct/j+vYz1yj4TQ@public.gmane.org

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