Re: iio_interrupt_trigger

[Matthias Meier <matthias.j.meier@gmx.net>]

Just for information...

I estimated the max ISR rate and sampling accuracy when using iio 
buffering triggered by a hardware PWM timer on a IMX6 dl board:

Up to a ISR rate of ~5kHz the accuracy of the sampling time is about 
+/-25us (measured over several seconds).

Above 10kHz, interrupts are lost (but CPU load still <25%).

Test conditions:

For this test I modified 'generic_buffer' so I could estimate the 
accuracy of the sampling point (by calculating min/max/mean of the time 
difference.
The kernel was 4.2.1 with CONFIG_PREEMPT=y
The ADC device I used is a AD7476 connected via SPI bus.
The scaling_governor was set to performance.
No additional load (buildroot rfs).
The results are about same when using a 4.1 TT-patched kernel with 
CONFIG_PREEMPT_RT_FULL=y (without special isr priority).

Conclusion: accurate high(er) speed sampling seems not to be possible 
this way (without dma buffered continuous sampling)

-- Matthias