iio: Add data_available callback for buffers

This patch adds a new data_available() callback to the iio_buffer_access_funcs
struct. The callback is used to indicate whether data is available in the buffer
for reading. It is meant to replace the stufftoread flag from the iio_buffer
struct. The reasoning for this is that the buffer implementation usually can
determine whether data is available rather easily based on its state, on the
other hand it can be rather tricky to update the stufftoread flag in a race free
way.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-buffer.c

@@ -37,6 +37,14 @@ static bool iio_buffer_is_active(struct iio_buffer *buf)
 	return !list_empty(&buf->buffer_list);
 }
 
+static bool iio_buffer_data_available(struct iio_buffer *buf)
+{
+	if (buf->access->data_available)
+		return buf->access->data_available(buf);
+
+	return buf->stufftoread;
+}
+
 /**
  * iio_buffer_read_first_n_outer() - chrdev read for buffer access
  *
@@ -70,7 +78,7 @@ unsigned int iio_buffer_poll(struct file *filp,
 		return -ENODEV;
 
 	poll_wait(filp, &rb->pollq, wait);
-	if (rb->stufftoread)
+	if (iio_buffer_data_available(rb))
 		return POLLIN | POLLRDNORM;
 	/* need a way of knowing if there may be enough data... */
 	return 0;

include/linux/iio/buffer.h

@@ -21,6 +21,8 @@ struct iio_buffer;
  * struct iio_buffer_access_funcs - access functions for buffers.
  * @store_to:		actually store stuff to the buffer
  * @read_first_n:	try to get a specified number of bytes (must exist)
+ * @data_available:	indicates whether data for reading from the buffer is
+ *			available.
  * @request_update:	if a parameter change has been marked, update underlying
  *			storage.
  * @get_bytes_per_datum:get current bytes per datum
@@ -43,6 +45,7 @@ struct iio_buffer_access_funcs {
 	int (*read_first_n)(struct iio_buffer *buffer,
 			    size_t n,
 			    char __user *buf);
+	bool (*data_available)(struct iio_buffer *buffer);
 
 	int (*request_update)(struct iio_buffer *buffer);