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 <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/industrialio-buffer.c

···
 		   iio_buffer_show_enable, iio_buffer_store_enable);
 static DEVICE_ATTR(watermark, S_IRUGO | S_IWUSR,
 		   iio_buffer_show_watermark, iio_buffer_store_watermark);
+static struct device_attribute dev_attr_watermark_ro = __ATTR(watermark,
+	S_IRUGO, iio_buffer_show_watermark, NULL);
 
 static struct attribute *iio_buffer_attrs[] = {
 	&dev_attr_length.attr,
···
 	memcpy(attr, iio_buffer_attrs, sizeof(iio_buffer_attrs));
 	if (!buffer->access->set_length)
 		attr[0] = &dev_attr_length_ro.attr;
+
+	if (buffer->access->flags & INDIO_BUFFER_FLAG_FIXED_WATERMARK)
+		attr[2] = &dev_attr_watermark_ro.attr;
 
 	if (buffer->attrs)
 		memcpy(&attr[ARRAY_SIZE(iio_buffer_attrs)], buffer->attrs,

include/linux/iio/buffer.h

···
 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
  * @read_first_n:	try to get a specified number of bytes (must exist)
···
  * @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
···
 	void (*release)(struct iio_buffer *buffer);
 
 	unsigned int modes;
+	unsigned int flags;
 };
 
 /**