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

From: Lars-Peter Clausen
Date: Wed May 13 2015 - 03:28:31 EST

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

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@xxxxxxxxx>
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

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