linux-kernel - Re: [PATCH RFC 1/3] iio: Add symlink to triggers in the device's trigger folder

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Date:	Wed, 13 May 2015 20:05:51 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Robert Dolca <robert.dolca@...il.com>,
	Jonathan Cameron <jic23@...nel.org>
CC:	Robert Dolca <robert.dolca@...el.com>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Hartmut Knaack <knaack.h@....de>,
	Peter Meerwald <pmeerw@...erw.net>,
	Denis CIOCCA <denis.ciocca@...com>
Subject: Re: [PATCH RFC 1/3] iio: Add symlink to triggers in the device's
 trigger folder

On 05/13/2015 08:03 PM, Robert Dolca wrote:
> On Wed, May 13, 2015 at 8:42 PM, Jonathan Cameron <jic23@...nel.org> wrote:
>>
>> On 13/05/15 08:28, Lars-Peter Clausen wrote:
>>> On 05/12/2015 09:06 PM, Jonathan Cameron wrote:
>>>> On 12/05/15 17:56, Lars-Peter Clausen wrote:
>>>>> On 05/08/2015 05:11 PM, Jonathan Cameron wrote:
>>>>>> On 16/04/15 05:01, Robert Dolca wrote:
>>>>>>> This patch adds a new function called iio_trigger_register_with_dev
>>>>>>> which is a wrapper for iio_trigger_register. Besides the iio_trigger
>>>>>>> struct this function requires iio_dev struct. It adds the trigger in
>>>>>>> the device's trigger list and saves a reference to the device in the
>>>>>>> trigger's struct.
>>>>>>>
>>>>>>> When the device is registered, in the trigger folder of the device
>>>>>>> (where current_trigger file resides) a symlink is being created for
>>>>>>> each trigger that was registered width iio_trigger_register_with_dev.
>>>>>>>
>>>>>>>     # ls -l /sys/bus/iio/devices/iio:device0/trigger/
>>>>>>> total 0
>>>>>>> -rw-r--r--    1 root     root          4096 Apr 16 08:33 current_trigger
>>>>>>> lrwxrwxrwx    1 root     root             0 Apr 16 08:33 trigger0 -> ../../trigg
>>>>>>> er0
>>>>>>>
>>>>>>> This should be used for device specific triggers. Doing this the user space
>>>>>>> applications can figure out what if the trigger registered by a specific device
>>>>>>> and what should they write in the current_trigger file. Currently some
>>>>>>> applications rely on the trigger name and this does not always work.
>>>>>>>
>>>>>>> This implementation assumes that the trigger is registered before the device is
>>>>>>> registered. If the order is not this the symlink will not be created but
>>>>>>> everything else will work as before.
>>>>>>>
>>>>>>> Signed-off-by: Robert Dolca <robert.dolca@...el.com>
>>>>>> I was rather hoping we'd get a few more comments on this.
>>>>>> In principle I like the idea, but it's new ABI and does make life
>>>>>> a tiny bit more complex, so what do people think?
>>>>>>
>>>>>> Few trivial code comments inline.
>>>>>
>>>>> I don't think it adds more information. Both the trigger and the
>>>>> device get registered for the same parent device, so you can already
>>>>> easily find the trigger for a device by going to the parent device
>>>>> and taking a look at the triggers registered by the parent device.
>>>> I had the same thought. The question is whether the slightly gain in
>>>> simplicity for userspace is worth it...  I'm undecided at the moment.
>>>
>>> As you may have guessed by now I'm always quite conservative when it
>>> comes to introducing new ABI. Simply because we have to maintain it
>>> forever, the less stuff to maintain forever the better.
>>>
>>> Hence I think all new ABI needs a compelling reason, e.g. like a
>>> improvement in performance. And of course this patch slightly
>>> simplifies things, but in my opinion not enough to justify a ABI
>>> extension. We can always find ways to simplify the interface, but the
>>> metric for ABI should be whether the simplification actually matters.
>>> In this case I don't think it does, finding the trigger for a device
>>> is not really hot-path. The amount of time saved will be disappear in
>>> the noise.
>>>
>>> And in my opinion applications shouldn't directly use the low-level
>>> ABI but rather use middle-ware libraries/frameworks, like e.g.
>>> libiio, and that's where you'd hide the complexities of a operation.
>>>
>>> - Lars
>> I'll go with Lars response on this one.  Not worth the hassle.
>> That's the nature of an RFC of course!
>>
>> Jonathan
>
> Would it be acceptable to add the symlinks without adding a new API?
> When a trigger is registered you could use the common parent to get a
> pointer to the iio_dev and then create the symlink. This is a little
> bit complicated but I think it can be done.

The concerns are with the symlink and with he symlink only. Adding new API 
inside the kernel is generally not as much of a problem as external ABI.

- Lars

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