linux-kernel - Re: [PATCH] staging: iio: trigger: Use standard attr for sampling frequency

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Date:	Wed, 18 Mar 2015 17:47:17 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Daniel Baluta <daniel.baluta@...el.com>,
	Jonathan Cameron <jic23@...nel.org>
CC:	Octavian Purdila <octavian.purdila@...el.com>,
	Hartmut Knaack <knaack.h@....de>,
	Peter Meerwald <pmeerw@...erw.net>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] staging: iio: trigger: Use standard attr for sampling
 frequency

On 03/18/2015 04:04 PM, Daniel Baluta wrote:
> On Sat, Mar 14, 2015 at 7:21 PM, Jonathan Cameron <jic23@...nel.org> wrote:
>> On 12/03/15 12:48, Daniel Baluta wrote:
>>> On Thu, Mar 12, 2015 at 10:40 AM, Lars-Peter Clausen <lars@...afoo.de> wrote:
>>>> On 03/12/2015 09:16 AM, Octavian Purdila wrote:
>>>>>
>>>>> On Wed, Mar 11, 2015 at 6:36 PM, Daniel Baluta <daniel.baluta@...el.com>
>>>>> wrote:
>>>>>>
>>>>>>
>>>>>> As written in Documentation/ABI/testing/sysfs-bus-iio the trigger
>>>>>> attribute for sampling frequency should be sampling_frequency.
>>>>>>
>>>>>> Fix this for iio-trig-periodic-rtc module in order to prepare it
>>>>>> for moving out of staging.
>>>>>>
>>>>>> Signed-off-by: Daniel Baluta <daniel.baluta@...el.com>
>>>>>> ---
>>>>>> Jonathan, this module is very useful for devices that do not have
>>>>>> an interrupt pin.
>>>>>>
>>>>>> We are working on drivers for such devices and would be very nice to
>>>>>> move this driver in advance to the IIO non-staging location.
>>>>>>
>>>>>> What do you say?
>>>>>>
>>>>>
>>>>> Hmm, I wonder what are the advantages of using RTC timers. Couldn't we
>>>>> use a regular kernel timer instead?
>>>>
>>>>
>>>> The long term plan is to get rid of the RTC timer trigger due to its various
>>>> limitations (poor resolution, etc).
>>>>
>>>> There is the hrtimer trigger
>>>> (https://github.com/analogdevicesinc/linux/blob/xcomm_zynq/drivers/staging/iio/trigger/iio-trig-hrtimer.c)
>>>> but we haven't agreed on a proper interface yet how to instantiate the
>>>> hrtimer trigger.
>>>>
>>>> Check the ml archive for the various discussions on it:
>>>> http://marc.info/?l=linux-iio&w=2&r=1&s=hrtimer&q=b
>>>
>>>
>>> Hi Lars,
>>>
>>> That was an interesting reading. There were people trying to push
>>>   hrtimer based IIO trigger 4 years ago :).
>>>
>>> I think it's now the time to have this upstream.
>>>
>>> I will be back :) (as many others said before) with an RFC patch.
>>>
>>> I think we should keep the following requirements:
>>>
>>> 1) Create a common framework for software based triggers.
>>> 2) User space driven configuration for trigger instances,
>>> as opposed to platform device files used for RTC based trigger
>>> 3) Remove RTC interrupt source, use hrtimers instead
>>>
>>> Still not clear, but I will trying to figure it out during implementation:
>>>
>>> 4) configfs vs sysfs interface.
>>>
>>> At the first glance, I would say we should stay with sysfs interface in order
>>> to avoid another dependency. But let's see how it works.
>> This issue with the sysfs only approach (as originally raised by Lars)
>> is that it is actually very poorly suited to instantiating new elements of
>> the device model.  Configfs was introduced in the first place exactly to
>> cover this area.  We only ended up with the instantiation code in
>> the sysfs trigger via sysfs because at the time (a good long while ago!)
>> I wasn't aware of configfs.
>>
>> I have some initial work on the base elements on an iio configfs interface
>> somewhere that I can dig out if you like.  I started working on it in a rare
>> quiet period about a year ago, but never got all that far.
>>
>> There aren't that many examples in tree of how to actually use configfs
>> so it's a bit more of a learning curve than sysfs!
>
> First notable problem with using configfs with IIO is boot time modules
> loading.
>
> Because,
>
> * configs uses module_init(configfs_init);
> * IIO uses subsys_initcall(iio_init);
>
> it is guaranteed that IIO will be loaded before configfs. Not fun! :)
>
> So for the moment I will not add configfs support directly into
> industrialiio-core but in a separate module.

Just fix configfs, that should clearly not be at the module init level.

But keeping it in a separate module doesn't hurt either way.

- Lars

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