iio: Add reference counting for buffers

Since the buffer is accessed by userspace we can not just free the buffers
memory once we are done with it in kernel space. There might still be open file
descriptors and userspace still might be accessing the buffer. This patch adds
support for reference counting to the IIO buffers. When a buffer is created and
initialized its initial reference count is set to 1. Instead of freeing the
memory of the buffer the buffer's _free() function will drop that reference
again. But only after the last reference to the buffer has been dropped the
buffer the buffer's memory will be freed. The IIO device will take a reference
to its primary buffer. The patch adds a small helper function for this called
iio_device_attach_buffer() which will get a reference to the buffer and assign
the buffer to the IIO device. This function must be used instead of assigning
the buffer to the device by hand. The reference is only dropped once the IIO
device is freed and we can be sure that there are no more open file handles. A
reference to a buffer will also be taken whenever the buffer is active to avoid
the buffer being freed while data is still being send to it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

1 files changed, 28 insertions, 0 deletions

diff --git a/include/linux/iio/buffer.h b/include/linux/iio/buffer.h
index a1124bdc4cac..6e428d96d570 100644
--- a/include/linux/iio/buffer.h
+++ b/include/linux/iio/buffer.h
@@ -11,6 +11,7 @@
 #define _IIO_BUFFER_GENERIC_H_
 #include <linux/sysfs.h>
 #include <linux/iio/iio.h>
+#include <linux/kref.h>
 
 #ifdef CONFIG_IIO_BUFFER
 
@@ -26,6 +27,8 @@ struct iio_buffer;
  * @set_bytes_per_datum:set number of bytes per datum
  * @get_length:		get number of datums in buffer
  * @set_length:		set number of datums in buffer
+ * @release:		called when the last reference to the buffer is dropped,
+ *			should free all resources allocated by the buffer.
  *
  * The purpose of this structure is to make the buffer element
  * modular as event for a given driver, different usecases may require
@@ -47,6 +50,8 @@ struct iio_buffer_access_funcs {
 	int (*set_bytes_per_datum)(struct iio_buffer *buffer, size_t bpd);
 	int (*get_length)(struct iio_buffer *buffer);
 	int (*set_length)(struct iio_buffer *buffer, int length);
+
+	void (*release)(struct iio_buffer *buffer);
 };
 
 /**
@@ -67,6 +72,7 @@ struct iio_buffer_access_funcs {
  * @demux_list:		[INTERN] list of operations required to demux the scan.
  * @demux_bounce:	[INTERN] buffer for doing gather from incoming scan.
  * @buffer_list:	[INTERN] entry in the devices list of current buffers.
+ * @ref:		[INTERN] reference count of the buffer.
  */
 struct iio_buffer {
 	int					length;
@@ -83,6 +89,7 @@ struct iio_buffer {
 	struct list_head			demux_list;
 	void					*demux_bounce;
 	struct list_head			buffer_list;
+	struct kref				ref;
 };
 
 /**
@@ -204,6 +211,24 @@ int iio_sw_buffer_preenable(struct iio_dev *indio_dev);
 bool iio_validate_scan_mask_onehot(struct iio_dev *indio_dev,
 	const unsigned long *mask);
 
+struct iio_buffer *iio_buffer_get(struct iio_buffer *buffer);
+void iio_buffer_put(struct iio_buffer *buffer);
+
+/**
+ * iio_device_attach_buffer - Attach a buffer to a IIO device
+ * @indio_dev: The device the buffer should be attached to
+ * @buffer: The buffer to attach to the device
+ *
+ * This function attaches a buffer to a IIO device. The buffer stays attached to
+ * the device until the device is freed. The function should only be called at
+ * most once per device.
+ */
+static inline void iio_device_attach_buffer(struct iio_dev *indio_dev,
+	struct iio_buffer *buffer)
+{
+	indio_dev->buffer = iio_buffer_get(buffer);
+}
+
 #else /* CONFIG_IIO_BUFFER */
 
 static inline int iio_buffer_register(struct iio_dev *indio_dev,
@@ -216,6 +241,9 @@ static inline int iio_buffer_register(struct iio_dev *indio_dev,
 static inline void iio_buffer_unregister(struct iio_dev *indio_dev)
 {}
 
+static inline void iio_buffer_get(struct iio_buffer *buffer) {}
+static inline void iio_buffer_put(struct iio_buffer *buffer) {}
+
 #endif /* CONFIG_IIO_BUFFER */
 
 #endif /* _IIO_BUFFER_GENERIC_H_ */