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 - 14:06:00 EST

On 05/13/2015 08:03 PM, Robert Dolca wrote:
On Wed, May 13, 2015 at 8:42 PM, Jonathan Cameron <jic23@xxxxxxxxxx> 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@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
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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