linux-kernel - Re: [PATCH v3 02/11] iio: core: register chardev only if needed

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Date:   Thu, 4 Feb 2021 17:17:48 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Alexandru Ardelean <alexandru.ardelean@...log.com>
CC:     <linux-kernel@...r.kernel.org>, <linux-iio@...r.kernel.org>,
        <lars@...afoo.de>, <Michael.Hennerich@...log.com>,
        <jic23@...nel.org>, <nuno.sa@...log.com>,
        <dragos.bogdan@...log.com>, <rafael@...nel.org>,
        <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v3 02/11] iio: core: register chardev only if needed

On Mon, 1 Feb 2021 16:50:56 +0200
Alexandru Ardelean <alexandru.ardelean@...log.com> wrote:

> We only need a chardev if we need to support buffers and/or events.
> 
> With this change, a chardev will be created only if an IIO buffer is
> attached OR an event_interface is configured.
> 
> Otherwise, no chardev will be created, and the IIO device will get
> registered with the 'device_add()' call.
> 
> 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 (slightly)
> fewer resources.
> 
> In order to not create a chardev, we mostly just need to not initialize the
> indio_dev->dev.devt field. If that is un-initialized, cdev_device_add()
> behaves like device_add().
> 
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.com>

Perhaps add a note to this description that it's 'sort' of a breaking ABI
change, but we don't believe it will affect any userspace code as they
likely check there is some point in opening the file before trying to do
so.

If we are wrong we can revert it but I doubt we are - subject to the normal
small fixable scripts people might be using and are happy to fix.

Jonathan

> ---
>  drivers/iio/industrialio-core.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> index 7db761afa578..0a6fd299a978 100644
> --- a/drivers/iio/industrialio-core.c
> +++ b/drivers/iio/industrialio-core.c
> @@ -1761,6 +1761,15 @@ static const struct file_operations iio_buffer_fileops = {
>  	.release = iio_chrdev_release,
>  };
>  
> +static const struct file_operations iio_event_fileops = {
> +	.owner = THIS_MODULE,
> +	.llseek = noop_llseek,
> +	.unlocked_ioctl = iio_ioctl,
> +	.compat_ioctl = compat_ptr_ioctl,
> +	.open = iio_chrdev_open,
> +	.release = iio_chrdev_release,
> +};
> +
>  static int iio_check_unique_scan_index(struct iio_dev *indio_dev)
>  {
>  	int i, j;
> @@ -1788,6 +1797,7 @@ static const struct iio_buffer_setup_ops noop_ring_setup_ops;
>  
>  int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
>  {
> +	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
>  	int ret;
>  
>  	if (!indio_dev->info)
> @@ -1805,9 +1815,6 @@ int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
>  	if (ret < 0)
>  		return ret;
>  
> -	/* configure elements for the chrdev */
> -	indio_dev->dev.devt = MKDEV(MAJOR(iio_devt), indio_dev->id);
> -
>  	iio_device_register_debugfs(indio_dev);
>  
>  	ret = iio_buffer_alloc_sysfs_and_mask(indio_dev);
> @@ -1836,9 +1843,15 @@ 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 (iio_dev_opaque->event_interface)
> +		cdev_init(&indio_dev->chrdev, &iio_event_fileops);
>  
> -	indio_dev->chrdev.owner = this_mod;
> +	if (indio_dev->buffer || iio_dev_opaque->event_interface) {
> +		indio_dev->dev.devt = MKDEV(MAJOR(iio_devt), indio_dev->id);
> +		indio_dev->chrdev.owner = this_mod;
> +	}
>  
>  	ret = cdev_device_add(&indio_dev->chrdev, &indio_dev->dev);
>  	if (ret < 0)