Re: [PATCH 1/3] iio: adc: hi-843x: Holt HI-8435/8436/8437 descrete ADC

From: Vladimir Barinov
Date: Thu Jun 18 2015 - 17:42:05 EST

Hi Lars,

Thank you for the review.

On 18.06.2015 22:33, Lars-Peter Clausen wrote:
On 06/07/2015 06:11 PM, Jonathan Cameron wrote:
On 01/06/15 13:20, Vladimir Barinov wrote:
Add Holt descrete ADC driver for HI-8435/8436/8437 chips

Signed-off-by: Vladimir Barinov <vladimir.barinov@xxxxxxxxxxxxxxxxxx>
Hmm. The main issue here is one man's discrete ADC is another man's
configurable general purpose input device.

The term discrete ADC is a bit ambiguous and I'm not even sure if this is the right term for this kind of device.

I'd call this a threshold detector. The device seems to have two comparators for each channel, one for the lower threshold, one for the upper threshold. If the voltage level goes above the upper threshold a FF is set, if it goes below the lower threshold the FF is cleared. Both transitions happen asynchronously as soon has the signal is below/above the threshold. And while converts a analog signal to digital one this is not what you typically call a ADC.

Should this be a separate/new directory in the drivers/iio/ for such driver?
What the type of iio_chan_spec should I use instead of IIO_VOLTAGE?

I wonder if we want to take this oportunity to add 1 bit packing to the
demux etc in the IIO core so we can have tighter packing on these
values. Shouldn't be too hard to do and we probably do want it if we are
going to support these sorts of devices.

Will take a bit of shuffling to pack the relevant channels together if only
a subset are enabled and to notice when no repacking at all is needed.
This will probably first one implementing in the core and pushing out into
the dummy driver to allow for testing of corner cases.

Yeah, the bit shuffling gets quite cumbersome and potentially expensive. I think we should try to avoid it if at least one of the channels in the same bank is enabled all of them are read. And then let userspace figure out which bits it wants to use.

But how exactly is the typical expect usage of this device. Like how would a userspace application use it? Is buffered mode where samples are taken in a continuous mode something that is really needed?
I was expecting to use triggered buffer for this device:
1) setup threshold levels via sysfs
2) enable scan elements
3) setup trigger
4) grab data from triggered iio buffer like the tools/iio/generic_buffer.c does, f.e.
./generic_buffer -n hi-8435 -t irqtrig0 -l 100 -c 1000

Actually I understand that I can just read manually the /sysfs/.../in_voltageXX_raw (or new/other name) values but using of iio generic irq trigger would be very good.

About bits shuffling/separating. I do think we can use banks byte length for one iio channel instead of 1-bit length to avoid such complexity.
Then let user space separate bank's channels by itself.


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