iio: Add support for indicating fixed watermarks

For buffers which have a fixed wake-up watermark the watermark attribute
should be read-only. Add a new FIXED_WATERMARK flag to the
struct iio_buffer_access_funcs, which can be set by a buffer
implementation.

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

include/linux/iio/buffer.h

@@ -17,6 +17,12 @@
 
 struct iio_buffer;
 
+/**
+ * INDIO_BUFFER_FLAG_FIXED_WATERMARK - Watermark level of the buffer can not be
+ *   configured. It has a fixed value which will be buffer specific.
+ */
+#define INDIO_BUFFER_FLAG_FIXED_WATERMARK BIT(0)
+
 /**
  * struct iio_buffer_access_funcs - access functions for buffers.
  * @store_to:		actually store stuff to the buffer
@@ -30,6 +36,7 @@ struct iio_buffer;
  * @release:		called when the last reference to the buffer is dropped,
  *			should free all resources allocated by the buffer.
  * @modes:		Supported operating modes by this buffer type
+ * @flags:		A bitmask combination of INDIO_BUFFER_FLAG_*
  *
  * The purpose of this structure is to make the buffer element
  * modular as event for a given driver, different usecases may require
@@ -54,6 +61,7 @@ struct iio_buffer_access_funcs {
 	void (*release)(struct iio_buffer *buffer);
 
 	unsigned int modes;
+	unsigned int flags;
 };
 
 /**