Date:	Wed, 15 Apr 2015 13:58:01 -0700
From:	Octavian Purdila <octavian.purdila@...el.com>
To:	Jonathan Cameron <jic23@...nel.org>
Cc:	Daniel Baluta <daniel.baluta@...el.com>,
	Joel Becker <jlbec@...lplan.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Hartmut Knaack <knaack.h@....de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	Paul Bolle <pebolle@...cali.nl>,
	Patrick Porlan <patrick.porlan@...el.com>,
	Adriana Reus <adriana.reus@...el.com>
Subject: Re: [PATCH v3 1/3] iio: configfs: Add configfs support to IIO

On Sun, Apr 12, 2015 at 8:59 AM, Jonathan Cameron <jic23@...nel.org> wrote:
> On 10/04/15 14:43, Daniel Baluta wrote:
>> On Thu, Apr 9, 2015 at 8:18 PM, Jonathan Cameron <jic23@...nel.org> wrote:
>>> On 08/04/15 14:30, Daniel Baluta wrote:
>>>> On Mon, Apr 6, 2015 at 5:18 PM, Daniel Baluta <daniel.baluta@...el.com> wrote:
>>>>> This module is the core of IIO configfs. It creates the "iio" subsystem under
>>>>> configfs mount point root, with one default group for "triggers".
>>>>>
>>>>> It offers basic interface for registering software trigger types. Next patches
>>>>> will add "hrtimer" and "sysfs" trigger types. To add a new trigger type we must
>>>>> create a new kernel module which implements iio_configfs_trigger.h interface.
>>>>>
>>>>> See Documentation/iio/iio_configfs.txt for more details on how configfs
>>>>> support for IIO works.
>>>>>
>>>>> Signed-off-by: Daniel Baluta <daniel.baluta@...el.com>
>>>>
>>>> Hi all,
>>>>
>>>> I also need your feedback on the following problem.
>>>>
>>>> We would like to be able to create hrtimer triggers from within
>>>> a kernel module. There are cases where we don't have an interrupt
>>>> pin or the interrupt pin is not connected, and we would like
>>>> that applications to run unmodified with these types of sensors too.
>>> A reasonable aim perhaps, as opposed to locally implemented polling
>>> all over the place (which should definitely not happen).
>>
>> Yes, exactly! :)
> I'm actually beginning to have my doubts about whether my initial
> response here was right.  Given your use cases and the complexity
> of matching frequencies, I think you are almost always better
> off implementing it in the drivers themselves (which can decide
> if there is new data or not).

I agree. Although I think having some helper functions in IIO core
should help keep the driver modifications to a minimum.

>>
>>>
>>> An alternative the zio guys used was to create timer
>>> based triggers on the fly purely based on them being requested
>>> (with an appropriate name IIRC)...  Doesn't quite solve your
>>> problem though as still needs userspace changes.
>>>>
>>>> We will split the current design into 3 parts:
>>>>
>>>> (1) IIO trigger handling generic part, which offers an API
>>>>      to register/unregister/get a reference to a trigger type
>>>>
>>>> (2) IIO configfs part that gets a reference to a trigger type and
>>>> handles user requests to create/destroy a trigger.
>>>>
>>>> (3) IIO hrtimer driver that use the API at (1) for registering
>>>> / deregistering a trigger type.
>>>>
>>>> Then, each driver in the case that it doesn't have a "real" trigger,
>>>> will get a reference to a "hrtimer" trigger type and create
>>>> a new "hrtimer" trigger.
>>>>
>>>> Does this look good to you? This could be easily done from
>>>> userspace, but we will need to modify our userspace applications.
>>> My initial thought is this really is a job for userspace, as should
>>> be in most cases connecting up the device specific trigger as well
>>> (as it's not always the right thing to use).
>>>
>>> In the general case it is far from obvious that an hrtimer is the
>>> right option.  Many usecases would be better off with a sysfs trigger
>>> or even running off a different interrupt based trigger entirely.
>>>>
>>>> Also, handling sampling_frequency/delay would be transparent to
>>>> applications if we provide this in kernel API.
>>> Not really as sampling frequency in this case should only be an
>>> attribute of the trigger, not the device.  We only really allow
>>> it for the device rather than always the triggers on the basis
>>> that it has impacts on other device elements (events etc).
>>
>> Well, if the trigger is directly created from the driver then we will
>> have a reference to a function that sets its delay.
>>
>> Each time the user sets the sampling frequency for the device
>> with hit write_raw and there on IIO_CHAN_INFO_SAMP_FREQ
>> case we also call set_delay(). Thus we always have have device
>> sampling frequency in sync with trigger delay.
>>
>> I know this sounds crazy :) and I don't like it. I am not sure how
>> to guarantee that device frequency is always in sync with trigger
>> delay.
> You can't that I can see.  Hence you've convinced me this is a bad
> idea.

I agree we can't accurately synchronize the device frequency with the
trigger frequency, but I think we can get it do a good enough level. I
also see no difference in accuracy in doing it from userspace or from
the driver itself.

Why do you think it is a bad idea to synchronize the trigger sample
frequency from the device sample frequency handler? I think that if we
want to provide the hrtimer trigger from the driver itself, we should
do this as well, otherwise there is no real gain to userspace.
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