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

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Date:   Tue, 17 Nov 2020 18:23:29 +0200
From:   Alexandru Ardelean <alexandru.ardelean@...log.com>
To:     <linux-iio@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:     <lars@...afoo.de>, <jic23@...nel.org>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>
Subject: [RFC PATCH 01/12] iio: core: register chardev only if needed

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>
---
 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)
-- 
2.17.1