Re: [RESEND RFC PATCH 0/2] Expose the PIO_ISR register on SAMA5D3

[Jonathan Cameron]

On 11/12/15 12:53, Linus Walleij wrote:
> Quoting extensively since I'm involving the linux-iio mailinglist.
> 
> The use case you describe is hand-in-glove with Industrial I/O.
> I think you want a trigger interface from IIO and read events from
> userspace using the IIO character device.
> 
> Look at the userspace examples in tools/iio for how it's used
> in userspace, the subsystem is in drivers/iio. I suspect
> drivers/iio/adc/polled-gpio.c or something is where you actually
> want to go with this. The module should do all the fastpath
> work and then expose what you actually want to know to
> userspace using the IIO triggers or events.
> 
> I have used IIO myself, it is really neat for this kind of usecase,
> and designed right from the ground up.
> 
> I think you whould think about how to write the right kind of
> driver for IIO to do what you want.
Peter has a spot of IIO experience as well :)

I'm not sure I entirely understand what the data flows are here so I may
get this completely wrong!

Sounds like a quick, dirty and simple 'capture unit' like you'd find on a PLC to
me (be bit one that doesn't grab much data - I use these all the time at
work to catch the output from beam break sensor on automated systems and
stuff like that).  Timers often support a copy to register on a gpio
signal but I'm not sure I've ever seen that supported in kernel either
(some discussion about doing this in IIO occurred a while ago but I don't
think anything ever came of it unfortunately). It was for the TI ECAP devices
by Matt Porter (cc'd)  Not that closely related but perhaps Matt will
have some insight here.

So:

Are we looking to synchronised control of the DAC
feeding the comparator or is that entirely autonomous?
(for now I'll assume autonomous - it gets interesting if
 not - we'd need the buffered output stuff Lars has for that)

How fast are we talking?

So I think we are basically looking for fast sampling of the gpio with latching.

I suspect the rates are high enough that an IIO trigger is going to be too expensive
(as it effectively runs as an irq).  That's fine though as they are optional if
you have a good reason not to use them and a direct polling of the isr and filling a
buffer might work.

We don't currently have 1 bit channel support in IIO and in this particular case
our normal buffers are going to be very inefficient as they are kfifo based
and hence will burn 1 byte per sample if we do this the simple way.
The closest we have gotten to a 1 bit support was a comparator driver and
in the end the author decided to support that via events which have way higher
overhead than I think you want.

So if IIO is the sensible way to support this I think we need something like
the following:

1) 1 bit data type support in IIO - not too bad to add, though will need
   to have some restrictions in the demux as arbitary bit channel recombining
   would be horrible and costly.  So in the first instance we'd probably burn 1 byte
   per 1 bit channel each sample - address this later perhaps.  If burning
   a byte, just specify that you have a channel with realbits = 1, storagebits = 8
   and it should all work.  I'd like to add 1 bit support fully if you are
   interested though!

2) A driver that can effectively check and clear the interrupt register and
   push that to the kfifo.  Probably running a kthread to keep the overhead
   low - something like the recent INA2XX driver is doing (though for a rather
   different reason).  That would then shove data into the buffer at regular
   intervals.

3) Normal userspace code would then read this - ideally with updates to
   correctly interpret it as boolean data.

Doesn't sound too bad - just a question of whether it will be lightweight
enough for your use case.

Assuming I have understood even vaguely what you are doing ;)
 
Sounds fun.

Jonathan
> 
> Yours,
> Linus Walleij
> 
> On Tue, Dec 8, 2015 at 4:20 AM, Peter Rosin <peda@xxxxxxxxxxxxxx> wrote:
>> From: Peter Rosin <peda@xxxxxxxxxx>
>>
>> Hi!
>>
>> I have a signal connected to a gpio pin which is the output of
>> a comparator. By changing the level of one of the inputs to the
>> comparator, I can detect the envelope of the other input to
>> the comparator by using a series of measurements much in the
>> same maner a manual ADC works, but watching for changes on the
>> comparator over a period of time instead of only the immediate
>> output.
>>
>> Now, the input signal to the comparator might have a high frequency,
>> which will cause the output from the comparator (and thus the GPIO
>> input) to change rapidly.
>>
>> A common(?) idiom for this is to use the interrupt status register
>> to catch the glitches, but then not have any interrupt tied to
>> the pin as that could possibly generate pointless bursts of
>> (expensive) interrupts.
>>
>> So, these two patches expose an interface to the PIO_ISR register
>> of the pio controllers on the platform I'm targetting. The first
>> patch adds some infrastructure to the gpio core and the second
>> patch hooks up "my" pin controller.
>>
>> But hey, this seems like an old problem and I was surprised that
>> I had to touch the source to do it. Which makes me wonder what I'm
>> missing and what others needing to see short pulses on a pin but not
>> needing/wanting interrupts are doing?
Basically a capture unit... Be it one that doesn't grab anything else
at the moment.
>>
>> Yes, there needs to be a way to select the interrupt edge w/o
>> actually arming the interrupt, that is missing. And probably
>> other things too, but I didn't want to do more work in case this
>> is a dead end for some reason...
>>
>> Cheers,
>> Peter
>>
>> Peter Rosin (2):
>>   gpio: Add isr property of gpio pins
>>   pinctrl: at91: expose the isr bit
>>
>>  Documentation/gpio/sysfs.txt   |   12 ++++++++++
>>  drivers/gpio/gpiolib-sysfs.c   |   30 ++++++++++++++++++++++++
>>  drivers/gpio/gpiolib.c         |   15 ++++++++++++
>>  drivers/pinctrl/pinctrl-at91.c |   50 ++++++++++++++++++++++++++++++++++++----
>>  include/linux/gpio/consumer.h  |    1 +
>>  include/linux/gpio/driver.h    |    2 ++
>>  6 files changed, 106 insertions(+), 4 deletions(-)
>>
>> --
>> 1.7.10.4
>>

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