linux-kernel - Re: [RFC PATCH 01/12] iio: core: register chardev only if needed

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Date:   Sat, 21 Nov 2020 18:02:46 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Alexandru Ardelean <alexandru.ardelean@...log.com>
Cc:     <linux-iio@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <lars@...afoo.de>
Subject: Re: [RFC PATCH 01/12] iio: core: register chardev only if needed

On Tue, 17 Nov 2020 18:23:29 +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>
I'll be honest. I have no idea why I didn't do this in first place!

I 'think' we are safe dropping this but I suppose it's possible some
odd code checks for the chrdev presence?

Hopefully not though.

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 419d6f8acc13..ca8b11541477 100644
> --- a/drivers/iio/industrialio-core.c
> +++ b/drivers/iio/industrialio-core.c
> @@ -1763,6 +1763,15 @@ static const struct file_operations iio_buffer_fileops = {
>  	.compat_ioctl = compat_ptr_ioctl,
>  };
>  
> +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;
> @@ -1790,6 +1799,7 @@ 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)
> @@ -1807,9 +1817,6 @@ 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);
> @@ -1838,9 +1845,15 @@ 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)