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, 9 insertions, 4 deletions

diff --git a/drivers/staging/iio/accel/sca3000_ring.c b/drivers/staging/iio/accel/sca3000_ring.c
index 0f2ee33..4a27beb 100644
--- a/drivers/staging/iio/accel/sca3000_ring.c
+++ b/drivers/staging/iio/accel/sca3000_ring.c
@@ -265,7 +265,7 @@ static struct iio_buffer *sca3000_rb_allocate(struct iio_dev *indio_dev)
 	return buf;
 }
 
-static inline void sca3000_rb_free(struct iio_buffer *r)
+static void sca3000_ring_release(struct iio_buffer *r)
 {
 	kfree(iio_to_hw_buf(r));
 }
@@ -274,23 +274,28 @@ static const struct iio_buffer_access_funcs sca3000_ring_access_funcs = {
 	.read_first_n = &sca3000_read_first_n_hw_rb,
 	.get_length = &sca3000_ring_get_length,
 	.get_bytes_per_datum = &sca3000_ring_get_bytes_per_datum,
+	.release = sca3000_ring_release,
 };
 
 int sca3000_configure_ring(struct iio_dev *indio_dev)
 {
-	indio_dev->buffer = sca3000_rb_allocate(indio_dev);
-	if (indio_dev->buffer == NULL)
+	struct iio_buffer *buffer;
+
+	buffer = sca3000_rb_allocate(indio_dev);
+	if (buffer == NULL)
 		return -ENOMEM;
 	indio_dev->modes |= INDIO_BUFFER_HARDWARE;
 
 	indio_dev->buffer->access = &sca3000_ring_access_funcs;
 
+	iio_device_attach_buffer(indio_dev, buffer);
+
 	return 0;
 }
 
 void sca3000_unconfigure_ring(struct iio_dev *indio_dev)
 {
-	sca3000_rb_free(indio_dev->buffer);
+	iio_buffer_put(indio_dev->buffer);
 }
 
 static inline