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

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Date:   Wed, 15 Apr 2020 13:56:13 +0000
From:   "Ardelean, Alexandru" <alexandru.Ardelean@...log.com>
To:     "jic23@...nel.org" <jic23@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>
Subject: Re: [PATCH v2] iio: core: register chardev only if needed

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@...log.com> 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@...log.com>
> > ---
> > 
> > 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.


> Thanks for tidying this up.
> 
> Jonathan
> 
> >  drivers/iio/industrialio-core.c | 17 +++++++++++++++--
> >  1 file changed, 15 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-
> > core.c
> > index f4daf19f2a3b..32e72d9fd1e9 100644
> > --- a/drivers/iio/industrialio-core.c
> > +++ b/drivers/iio/industrialio-core.c
> > @@ -1676,6 +1676,15 @@ static int iio_check_unique_scan_index(struct iio_dev
> > *indio_dev)
> >  
> >  static const struct iio_buffer_setup_ops noop_ring_setup_ops;
> >  
> > +static const struct file_operations iio_event_fileops = {
> > +	.release = iio_chrdev_release,
> > +	.open = iio_chrdev_open,
> > +	.owner = THIS_MODULE,
> > +	.llseek = noop_llseek,
> > +	.unlocked_ioctl = iio_ioctl,
> > +	.compat_ioctl = compat_ptr_ioctl,
> > +};
> > +
> >  int __iio_device_register(struct iio_dev *indio_dev, struct module
> > *this_mod)
> >  {
> >  	int ret;
> > @@ -1726,7 +1735,10 @@ int __iio_device_register(struct iio_dev *indio_dev,
> > struct module *this_mod)
> >  		indio_dev->setup_ops == NULL)
> >  		indio_dev->setup_ops = &noop_ring_setup_ops;
> >  
> > -	cdev_init(&indio_dev->chrdev, &iio_buffer_fileops);
> > +	if (indio_dev->buffer)
> > +		cdev_init(&indio_dev->chrdev, &iio_buffer_fileops);
> > +	else if (indio_dev->event_interface)
> > +		cdev_init(&indio_dev->chrdev, &iio_event_fileops);
> >  
> >  	indio_dev->chrdev.owner = this_mod;
> >  
> > @@ -1754,7 +1766,8 @@ EXPORT_SYMBOL(__iio_device_register);
> >   **/
> >  void iio_device_unregister(struct iio_dev *indio_dev)
> >  {
> > -	cdev_device_del(&indio_dev->chrdev, &indio_dev->dev);
> > +	if (indio_dev->buffer || indio_dev->event_interface)
> > +		cdev_device_del(&indio_dev->chrdev, &indio_dev->dev);
> >  
> >  	mutex_lock(&indio_dev->info_exist_lock);
> >