Re: help regarding Hardware Triggers in IIO ADC Driver

Re: help regarding Hardware Triggers in IIO ADC Driver

[Sricharan Chalasani]

Hi Jonathan and all,

I am really thankful to you for your quick responses. It really helps
me a lot to quckly understand how IIO works. Many thanks in advance
again.

This is in continuation to my previous email and your valuable
response, I have decided to use IIO RTC Trigger as a trigger provider
and write my own Generic ADC driver. Before start writing my own
Generic Driver, may I request your help to better understand how a
Trigger really works.

As an experiment, I will make use of IIO RTC Trigger
(staging/iio/trigger) as Trigger provider to Trigger Simple Dummy IIO
Driver (staging/iio).

Assuming that I have 2 channels on my ADC. Following is my basic
understanding of how an RTC Trigger works.

(After loading both IIO RTC Trigger driver and Simple Dummy IIO Driver)

Set up periodic RTC trigger frequency

#echo 1000 > /sys/devices/trigger0/frequency

Set up the channels in use

# echo 1 > /sys/bus/iio/devices/iio:device0/scan_elements/in_voltage0_en
# echo 1 > /sys/bus/iio/devices/iio:device0/scan_elements/in_voltage1_en

Set up the trigger we want to use (it must be the name of one of the
triggers present in iio/devices) (assuming that the rtc trigger name -
my_rtc_trig0)

# echo "my_rtc_trig0" > /sys/bus/iio/devices/iio:device0/trigger/current_trigger

Set up the buffer length (Maximum number of data kept in the buffer,
if we reach this number, the older data will be dropped)

# echo 100 > /sys/bus/iio/devices/iio:device0/buffer/length

Enable the capture

# echo 1 > /sys/bus/iio/devices/iio:device0/buffer/enable

With the above configuration, I understand that whenever the RTC
Periodic Interrupt is generated, the periodic RTC interrupt handler
(ISR) in-turn invokes my Simple Dummy Trigger Handler
(iio_simple_dummy_trigger_h). My Simple dummy trigger handler is
invoked (iio_simple_dummy_trigger_h) periodically with the help of RTC
periodic interrupt. I will copy data corresponding to all the enabled
channels (fake data in this case) to ring buffer from the function
iio_simple_dummy_trigger_h periodically.

Please correct me if my understanding of how a periodic RTC Trigger
really Triggers an IIO device to copy data to ring buffer
periodically, is wrong ......


Thanks in advance,

Sricharan.







On Fri, Aug 22, 2014 at 2:38 AM, Jonathan Cameron <jic23@kernel.org> wrote:
> On 20/08/14 19:19, Sricharan Chalasani wrote:
>> Hi Mr. Jonathan,
> Hi Sricharan,
>
> It is almost always best to address questions like this to the relevant mailing list
> (find that out from MAINTAINERS in the root of the kernel source tree).  In this
> case linux-iio.  If nothing else, others on there tend to respond faster than I do
> and it is useful to have discussions such as this available in the archives to
> refer to later.
>>
>> My name is Sricharan. I am from Pune, South India. I know it is not correct to directly write to you. But I request you
>> to please help me in this regard.
>>
>> I tried a lot to undersatand whether my following approach really satisfy the principles of IIO Subsystem, but feel like
>> I am missing something in properly understanding my Subsystem properly.
>>
>> In the existing IIO drivers (linux-3.16), my understanding is that Hardware Triggers are just used to copy sampled data
>> from the ADC to the Kernel buffer. I think none of the hardware triggers are really configuring ADC to Start the analog
>> to digital conversion.
> Actually quite a few hardware triggers can be used in this fashion, though often in the case
> of Data Ready type interrupts they can be used to trigger 'other' devices.
> Also we have a number of standalone triggers. The most relevant are still to be
> found in drivers/staging/iio/triggers (rtc and blackfin timers triggers).  They
> are there because we actually intend to drop them once we have a better means of
> configuring the high resolution timer based trigger that has been posted to the
> lists a few times.  The jitter on that trigger is actually very low reducing the
> point of using hardware counters and now high resolution timers are nearly always
> available.
>>
>> I have a Hardware Counter on my Board. I am planning to configure my Counter overflow interrupt as an IIO Hardware
>> Trigger.
> This is pretty much a periodic rtc interrupt (note these are now all emulated via high
> res timers in the rtc system but they used to do it exactly the way you describe!)
>
>> In my IIO ADC driver probe function I am planning to start my Counter for the first time. Once the counter
>> overflow interrupted is generated, inside the counter ISR, I am planning to configure ADC to start analog to digital
>> conversion on all the channels (4 of them), one sample on each channel and once the Conversion is completed, I am
>> planning to copy the Sampled data to Kernel buffer associated with the Trigger.
> This is fine and is how pretty much all the buffer supporting drivers that don't
> supply a trigger (no fixed internal sampling rate typically) do it.
> A simple example would be something like drivers/iio/max1363 though in that conversions
> are clocked directly from the i2c bus so 'appear' instantaneous.
>
> If you have an interrupt to indicate the end of the conversion, then the normal trick is
> to use a completion to basically sleep the trigger handler (which is in a thread) until
> the data is ready.  Lots of drivers do this as well.
>
>>
>> I will program the Counter again and wait for interrupt and repeat the steps above. I am planning to continue this
>> approach indefinitely.
> Hmm.  If you actually have to reprogram the counter rather than it being periodic you 'might'
> have a small amount of drift if you are not careful...
>>
>> Does this approach satisfy the principles of IIO Subsystem ? Am I missing anything here in understanding the IIO
>> Subsystem properly ?
> This is fine.  Note however that what you describe is best implemented as two drivers.
> One is a generic adc driver which uses the trigger and the second is a trigger provider.
>
> J
>>
>> Thanks in advance,
>>
>> Sricharan,
>> Pune (India).
>>
>>

Re: help regarding Hardware Triggers in IIO ADC Driver

[Jonathan Cameron]

On 25/08/14 09:08, Sricharan Chalasani wrote:
> Hi Jonathan and all,
> 
> I am really thankful to you for your quick responses. It really helps
> me a lot to quckly understand how IIO works. Many thanks in advance
> again.
> 
> This is in continuation to my previous email and your valuable
> response, I have decided to use IIO RTC Trigger as a trigger provider
> and write my own Generic ADC driver. Before start writing my own
> Generic Driver, may I request your help to better understand how a
> Trigger really works.
> 
> As an experiment, I will make use of IIO RTC Trigger
> (staging/iio/trigger) as Trigger provider to Trigger Simple Dummy IIO
> Driver (staging/iio).
> 
> Assuming that I have 2 channels on my ADC. Following is my basic
> understanding of how an RTC Trigger works.
> 
> (After loading both IIO RTC Trigger driver and Simple Dummy IIO Driver)
> 
> Set up periodic RTC trigger frequency
> 
> #echo 1000 > /sys/devices/trigger0/frequency
> 
> Set up the channels in use
> 
> # echo 1 > /sys/bus/iio/devices/iio:device0/scan_elements/in_voltage0_en
> # echo 1 > /sys/bus/iio/devices/iio:device0/scan_elements/in_voltage1_en
> 
> Set up the trigger we want to use (it must be the name of one of the
> triggers present in iio/devices) (assuming that the rtc trigger name -
> my_rtc_trig0)
> 
> # echo "my_rtc_trig0" > /sys/bus/iio/devices/iio:device0/trigger/current_trigger
> 
> Set up the buffer length (Maximum number of data kept in the buffer,
> if we reach this number, the older data will be dropped)
> 
> # echo 100 > /sys/bus/iio/devices/iio:device0/buffer/length
> 
> Enable the capture
> 
> # echo 1 > /sys/bus/iio/devices/iio:device0/buffer/enable
> 
> With the above configuration, I understand that whenever the RTC
> Periodic Interrupt is generated, the periodic RTC interrupt handler
> (ISR) in-turn invokes my Simple Dummy Trigger Handler
> (iio_simple_dummy_trigger_h). My Simple dummy trigger handler is
> invoked (iio_simple_dummy_trigger_h) periodically with the help of RTC
> periodic interrupt. I will copy data corresponding to all the enabled
> channels (fake data in this case) to ring buffer from the function
> iio_simple_dummy_trigger_h periodically.
A small point - it's a fifo not a ring buffer, but otherwise fine
(used to be a ring buffer, but no one really cared about that so
we switched over the a fifo using the kfifo infrastructure.)
> 
> Please correct me if my understanding of how a periodic RTC Trigger
> really Triggers an IIO device to copy data to ring buffer
> periodically, is wrong ......
All correct as far as I can see.

J
> 
> 
> Thanks in advance,
> 
> Sricharan.
> 
> 
> 
> 
> 
> 
> 
> On Fri, Aug 22, 2014 at 2:38 AM, Jonathan Cameron <jic23@kernel.org> wrote:
>> On 20/08/14 19:19, Sricharan Chalasani wrote:
>>> Hi Mr. Jonathan,
>> Hi Sricharan,
>>
>> It is almost always best to address questions like this to the relevant mailing list
>> (find that out from MAINTAINERS in the root of the kernel source tree).  In this
>> case linux-iio.  If nothing else, others on there tend to respond faster than I do
>> and it is useful to have discussions such as this available in the archives to
>> refer to later.
>>>
>>> My name is Sricharan. I am from Pune, South India. I know it is not correct to directly write to you. But I request you
>>> to please help me in this regard.
>>>
>>> I tried a lot to undersatand whether my following approach really satisfy the principles of IIO Subsystem, but feel like
>>> I am missing something in properly understanding my Subsystem properly.
>>>
>>> In the existing IIO drivers (linux-3.16), my understanding is that Hardware Triggers are just used to copy sampled data
>>> from the ADC to the Kernel buffer. I think none of the hardware triggers are really configuring ADC to Start the analog
>>> to digital conversion.
>> Actually quite a few hardware triggers can be used in this fashion, though often in the case
>> of Data Ready type interrupts they can be used to trigger 'other' devices.
>> Also we have a number of standalone triggers. The most relevant are still to be
>> found in drivers/staging/iio/triggers (rtc and blackfin timers triggers).  They
>> are there because we actually intend to drop them once we have a better means of
>> configuring the high resolution timer based trigger that has been posted to the
>> lists a few times.  The jitter on that trigger is actually very low reducing the
>> point of using hardware counters and now high resolution timers are nearly always
>> available.
>>>
>>> I have a Hardware Counter on my Board. I am planning to configure my Counter overflow interrupt as an IIO Hardware
>>> Trigger.
>> This is pretty much a periodic rtc interrupt (note these are now all emulated via high
>> res timers in the rtc system but they used to do it exactly the way you describe!)
>>
>>> In my IIO ADC driver probe function I am planning to start my Counter for the first time. Once the counter
>>> overflow interrupted is generated, inside the counter ISR, I am planning to configure ADC to start analog to digital
>>> conversion on all the channels (4 of them), one sample on each channel and once the Conversion is completed, I am
>>> planning to copy the Sampled data to Kernel buffer associated with the Trigger.
>> This is fine and is how pretty much all the buffer supporting drivers that don't
>> supply a trigger (no fixed internal sampling rate typically) do it.
>> A simple example would be something like drivers/iio/max1363 though in that conversions
>> are clocked directly from the i2c bus so 'appear' instantaneous.
>>
>> If you have an interrupt to indicate the end of the conversion, then the normal trick is
>> to use a completion to basically sleep the trigger handler (which is in a thread) until
>> the data is ready.  Lots of drivers do this as well.
>>
>>>
>>> I will program the Counter again and wait for interrupt and repeat the steps above. I am planning to continue this
>>> approach indefinitely.
>> Hmm.  If you actually have to reprogram the counter rather than it being periodic you 'might'
>> have a small amount of drift if you are not careful...
>>>
>>> Does this approach satisfy the principles of IIO Subsystem ? Am I missing anything here in understanding the IIO
>>> Subsystem properly ?
>> This is fine.  Note however that what you describe is best implemented as two drivers.
>> One is a generic adc driver which uses the trigger and the second is a trigger provider.
>>
>> J
>>>
>>> Thanks in advance,
>>>
>>> Sricharan,
>>> Pune (India).
>>>
>>>
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