Re: spi_async and heavy CPU occupation by ksoftirqd

[David Brownell <david-b-yBeKhBN/0LDR7s880joybQ@public.gmane.org>]

On Wednesday 08 August 2007, llandre wrote:
> 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.

That's excessive.


> 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?

Are your transfers large enough to kick in DMA, or is it doing PIO?

At a first glance, I wonder if you're not spending too much time
in the spi_imx "pump_transfers" tasklet.  I have no idea how well
tuned that driver is...

I'd expect that using spi_sync() wouldn't save much, but it might
be worth verifying that.

- Dave

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