[RFC PATCH 05/14] iio: core: register chardev only if needed

[Alexandru Ardelean <alexandru.ardelean@analog.com>]

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 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.

What's important to remember, is that removing a chardev also requires that
'indio_dev->dev.devt' to be initialized. If it is, a '/dev/iio:deviceX'
file will be created by the kernel, regardless of whether it has a chardev
attached to it or not.
If that field is not initialized, cdev_device_{add,del}() behave{s} like
device_{add,del}().

Also, since we plan to add more chardevs for IIO buffers, we need to
consider now a separate IDA object just for chardevs.
Currently, the only benefit is that chardev IDs will be continuous.
However, when adding a chardev per IIO buffer, the IDs for the chardev
could outpace the IDs for IIO devices, so these should be decoupled.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
---
 drivers/iio/industrialio-core.c | 58 +++++++++++++++++++++++++++++----
 1 file changed, 51 insertions(+), 7 deletions(-)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index 32c489139cd2..d74279efeca4 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -35,6 +35,9 @@
 /* IDA to assign each registered device a unique id */
 static DEFINE_IDA(iio_ida);
 
+/* IDA to assign each registered character device a unique id */
+static DEFINE_IDA(iio_chrdev_ida);
+
 static dev_t iio_devt;
 
 #define IIO_DEV_MAX 256
@@ -1680,8 +1683,40 @@ static int iio_check_unique_scan_index(struct iio_dev *indio_dev)
 	return 0;
 }
 
+static int iio_device_alloc_chrdev_id(struct device *dev)
+{
+	int id;
+
+	id = ida_simple_get(&iio_chrdev_ida, 0, 0, GFP_KERNEL);
+	if (id < 0) {
+		/* cannot use a dev_err as the name isn't available */
+		dev_err(dev, "failed to get device id\n");
+		return id;
+	}
+
+	dev->devt = MKDEV(MAJOR(iio_devt), id);
+
+	return 0;
+}
+
+static void iio_device_free_chrdev_id(struct device *dev)
+{
+	if (!dev->devt)
+		return;
+	ida_simple_remove(&iio_chrdev_ida, MINOR(dev->devt));
+}
+
 static const struct iio_buffer_setup_ops noop_ring_setup_ops;
 
+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,
+};
+
 int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
 {
 	int ret;
@@ -1701,9 +1736,6 @@ 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);
@@ -1732,16 +1764,27 @@ 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;
+	if (indio_dev->buffer || indio_dev->event_interface) {
+		indio_dev->chrdev.owner = this_mod;
+
+		ret = iio_device_alloc_chrdev_id(&indio_dev->dev);
+		if (ret)
+			goto error_unreg_eventset;
+	}
 
 	ret = cdev_device_add(&indio_dev->chrdev, &indio_dev->dev);
-	if (ret < 0)
-		goto error_unreg_eventset;
+	if (ret)
+		goto error_free_chrdev_id;
 
 	return 0;
 
+error_free_chrdev_id:
+	iio_device_free_chrdev_id(&indio_dev->dev);
 error_unreg_eventset:
 	iio_device_unregister_eventset(indio_dev);
 error_free_sysfs:
@@ -1761,6 +1804,7 @@ EXPORT_SYMBOL(__iio_device_register);
 void iio_device_unregister(struct iio_dev *indio_dev)
 {
 	cdev_device_del(&indio_dev->chrdev, &indio_dev->dev);
+	iio_device_free_chrdev_id(&indio_dev->dev);
 
 	mutex_lock(&indio_dev->info_exist_lock);
 
-- 
2.17.1