Support high speed sampling on i.MX6ul ADCs

[markfeathers]

Test with example high speed ADC:
https://community.nxp.com/t5/i-MX-Processors/An-adc-using-an-imx6ull-processor-enables-continuous-acquisition/m-p/1719805

[ts-kris]

Looks like this is a module in both 5.10 and 6.6, in both full and minimal defconfigs, iio-trig-sysfs

Do we want to make it a builtin, or just document its use?

[markfeathers]

Looks like I missed that, but we should test & document to get the higher speed ADC support.

[markfeathers]

Took a quick swing at this while testing another i.MX6ul ADC issue. This is sort of working, but has a surprising side effect. It only supports a single channel.

# Add trigger device
modprobe iio-trig-sysfs
echo 0 > /sys/bus/iio/devices/iio_sysfs_trigger/add_trigger
echo "206250" > /sys/bus/iio/devices/iio\:device2/in_voltage_sampling_frequency
iio_readdev -t trigger0 -s 1024 -b 1024 iio:device2 voltage0 > samples.bin

The iio interface should permit us to do this:

iio_readdev -t trigger0 -s 1024 -b 1024 iio:device2 voltage0 voltage1 voltage5 voltage8 voltage9 > samples.bin

But this fails with:

Unable to allocate buffer: Invalid argument (22)

Ultimately right now we cannot enable multiple channels as scan elements and I'm not sure why:

root@tsimx6ul:~# echo 1 > /sys/bus/iio/devices/iio\:device2/scan_elements/in_voltage0_en 
root@tsimx6ul:~# echo 1 > /sys/bus/iio/devices/iio\:device2/scan_elements/in_voltage1_en 
-bash: echo: write error: Invalid argument