Re: ZIO wiki analysis of IIO and why it doesn't meet there requirements [was Re: Industrial Input Output analysis]

[Alessandro Rubini <rubini@gnudd.com>]

Hello Lars-Peter, sorry for the delay.
You raise different points than Jonathan did, I hope I'm not repeating
stuff Federico already said.

>> 2) Output signal support is patchy and currently unbuffered, but there
>> is some.
 
> Support for output buffers and trigger output is on my TODO list and will
> probably work on it soon.

Good. We'll evaluate it, when available. Thanks.  Time for me to subscribe
to the list, I think...

BTW: when I added output support to zio it was a matter of two hours
(it was unexpected, but the design proved solid).

> I agree, some of the metadata fields look like they should not be in there,
> like minor, major, devicename, ....
> As Jonathan says all this information is already available to the
> application processing the datastream. And the only usecase for having the
> metadata interleaved with the real data is that you just want to dump the
> stream right onto storage without further processing.

Exactly. We have thousands of I/O boards, some PC some PowerPC (thus
the endianness information in the control block), and some even may be
remote -- hostless I/O cards using etherbone
(ohwr.org/projects/etherbone-core).

> I'm wondering how this bulk mode in ZIO works. Does the hardware simply
> generate a bunch of samples after a trigger event, or do you have a buffer
> inside the hardware and it periodically takes samples and you flush the
> hardware buffer on a trigger event?

The cards are serious fpga engines. They have 256MB (or more, or less)
internal RAM to store the actual acquisition, which is later pushed to
the host in big blocks.  We also expect to have a scope-like environment,
where the threshold is actually the trigger event, and we have pre-samples
being sent to the PC -- the FPGA will use the RAM circularly, monitoring
data for the trigger event.

> I wonder if it makes sense to add the concept of timestamp source. A
> timestamp source would be associated with an trigger and whenever it is
> triggered the timestamp would be read and made available for further
> processing by the buffer. One timestamp source would be our nanosecond
> clock, others could be generated by external hardware.

I think it makes sense. For us, the stamp comes from hardware because
the device knows the current time at sub-nanosecond precision, moreover,
the trigger happens much earlier than the DMA is performed -- or
much later for output.

> I still think that ZIO is just a subset of IIO.

I don't think it's a subset: it has a different scope. As said, I don't
want to run zio in small systems, where iio fits well.

> We do use IIO for high speed data conversion.

May I know how high?

Sure I want to run my ad7888 with a 10MHz spi and double buffering
(when federico gets to write the iio code), but that's 625kS to be
shared among the channels -- and I've already some concern about
processing them one at a a time.  We have 640 times as many (4 *
100MS); even if it's burst (not continuous). I doubt I can push them to
the buffers sample-by-sample, and in this case the overhead of our
half-a-kB-per-channel meta information is negligible.

> The evaluation on the ZIO wiki reads a bit as if it is assumed that IIO is
> set in stone.

Yes. That's a problem when you must evaluate something. Federico
started with 2.6.39, actually, where sysfs was much more painful,
and we are happy it's getting better and better.

> Though I think it should be possible to easily adopt it to the ZIO
> needs.

Let's see it over time.  Maybe you are right, or maybe not. Some
design choices can't be changed: you need internal consistency in a
subsystem.  Sure push_block would help, but aren't your buffers
sample-oriented anyways? (a genuine question, not rethoric -- I didn't
personally study the internals to fine details, not yet).

> If we keep two different frameworks there will be a huge overlap
> in functionality and also supported devices.

Overlap happens. I don't see it as a problem. You overlap with
linux-sensors already, and everything overlaps with generic char
devices.

As for devices, you are partly right. Sure we are starting with the
same devices that you are using, but the big ones (our target) may be
zio-only. I have no problems in keeping the AD7* zio drivers
off-the-kernel, since that's iio's use case -- actually we only
expect to only have a few of them, mainly as examples.

/alessandro