iio: Add support for blocking IO on buffers

Currently the IIO buffer interface only allows non-blocking reads. This patch
adds support for blocking IO. In blocking mode the thread will go to sleep if no
data is available and will wait for the buffer implementation to signal that new
data is available by waking up the buffers waitqueue.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-buffer.c

@@ -56,13 +56,34 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
 {
 	struct iio_dev *indio_dev = filp->private_data;
 	struct iio_buffer *rb = indio_dev->buffer;
+	int ret;
 
 	if (!indio_dev->info)
 		return -ENODEV;
 
 	if (!rb || !rb->access->read_first_n)
 		return -EINVAL;
-	return rb->access->read_first_n(rb, n, buf);
+
+	do {
+		if (!iio_buffer_data_available(rb)) {
+			if (filp->f_flags & O_NONBLOCK)
+				return -EAGAIN;
+
+			ret = wait_event_interruptible(rb->pollq,
+					iio_buffer_data_available(rb) ||
+					indio_dev->info == NULL);
+			if (ret)
+				return ret;
+			if (indio_dev->info == NULL)
+				return -ENODEV;
+		}
+
+		ret = rb->access->read_first_n(rb, n, buf);
+		if (ret == 0 && (filp->f_flags & O_NONBLOCK))
+			ret = -EAGAIN;
+	 } while (ret == 0);
+
+	return ret;
 }
 
 /**