Re: [PATCH v2] iio: core: register chardev only if needed

[Lars-Peter Clausen]

On 4/15/20 3:56 PM, Ardelean, Alexandru wrote:
> On Tue, 2020-04-14 at 19:06 +0100, Jonathan Cameron wrote:
> > [External]
> >
> > On Tue, 14 Apr 2020 11:36:56 +0300
> > Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx> wrote:
> >
> > > The final intent is to localize all buffer ops into the
> > > industrialio-buffer.c file, to be able to add support for multiple buffers
> > > per IIO device.
> > >
> > > We only need a chardev if we need to support buffers and/or events.
> > >
> > > With this change, a chardev will be created:
> > > 1. if there is an IIO buffer attached OR
> > > 2. if there is an event_interface configured
> > >
> > > Otherwise, no chardev will be created.
> > > Quite a lot of IIO devices don't really need a chardev, so this is a minor
> > > improvement to the IIO core, as the IIO device will take up fewer
> > > resources.
> > >
> > > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>
> > > ---
> > >
> > > Changelog v1 -> v2:
> > > * split away from series 'iio: core,buffer: re-organize chardev creation';
> > >    i'm getting the feeling that this has some value on it's own;
> > >    no idea if it needs 'Fixes' tag; it is a bit fuzzy to point to a patch
> > >    which this would be fixed by this; i'm guessing it would be fine
> > >    without one
> > I'd argue it's an 'optimization' rather than a fix :)
> >
> > Still looks good to me but I'd like it to sit for a little while to
> > see if anyone points out something we are both missing!
> This is not good.
> It seems that I did not properly test all cases.
> I had to break a device to not have an event_interface to notice that the sysfs
> doesn't get instantiated either because device_add is missing.
>
> Will do another try.

I think you also have to make the `indio_dev->dev.devt = ...` 
conditional. Or conditionally use device_add() instead of device_add_cdev().

If you go for the former you need to call cdev_device_del() 
unconditionally, for the latter call device_del() or cdev_device_del() 
depending on whether the cdev was registered.

- Lars