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 18:42:50 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Lars-Peter Clausen <lars@...afoo.de>,
	Robert Dolca <robert.dolca@...el.com>,
	linux-iio@...r.kernel.org
CC:	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 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

> 
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