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

From: Alexandru Ardelean
Date: Thu Dec 03 2020 - 04:55:06 EST


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@xxxxxxxxxx>
---

Changelog v1 -> v2:
* https://lore.kernel.org/linux-iio/20201117162340.43924-2-alexandru.ardelean@xxxxxxxxxx/
* split away from series; I don't know when I will have time to re-visit
the entire original series, so might as well move forward a few patches

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 c2e4c267c36b..d4de32d878aa 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -1725,6 +1725,15 @@ 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;
@@ -1752,6 +1761,7 @@ 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)
@@ -1769,9 +1779,6 @@ 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);
@@ -1800,9 +1807,15 @@ 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)
--
2.27.0