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

[Lars-Peter Clausen]

On 05/13/2015 08:05 PM, Lars-Peter Clausen wrote:
> 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.

Sorry, I should have added you can easily find out which triggers and which 
devices belong together by basically doing a dirname `readlink 
/sys/bus/iio/devices/X`. Those that are at the same place in the global 
hierarchy belong to the same device.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/