iio: buffer: Expose data available

Add a sysfs attribute that exposes buffer data available to userspace.
This attribute can be checked at runtime to determine the overall buffer
fill level (across all allocated buffers).
Signed-off-by: Matt Fornero <matt.fornero@mathworks.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -1413,6 +1413,16 @@ Description:
 		the available samples after the timeout expires and thus have a
 		maximum delay guarantee.
 
+What:		/sys/bus/iio/devices/iio:deviceX/buffer/data_available
+KernelVersion: 4.16
+Contact:	linux-iio@vger.kernel.org
+Description:
+		A read-only value indicating the bytes of data available in the
+		buffer. In the case of an output buffer, this indicates the
+		amount of empty space available to write data to. In the case of
+		an input buffer, this indicates the amount of data available for
+		reading.
+
 What:		/sys/bus/iio/devices/iio:deviceX/buffer/hwfifo_enabled
 KernelVersion: 4.2
 Contact:	linux-iio@vger.kernel.org

drivers/iio/industrialio-buffer.c

@@ -1198,6 +1198,18 @@ static ssize_t iio_buffer_store_watermark(struct device *dev,
 	return ret ? ret : len;
 }
 
+static ssize_t iio_dma_show_data_available(struct device *dev,
+						struct device_attribute *attr,
+						char *buf)
+{
+	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
+	size_t bytes;
+
+	bytes = iio_buffer_data_available(indio_dev->buffer);
+
+	return sprintf(buf, "%zu\n", bytes);
+}
+
 static DEVICE_ATTR(length, S_IRUGO | S_IWUSR, iio_buffer_read_length,
 		   iio_buffer_write_length);
 static struct device_attribute dev_attr_length_ro = __ATTR(length,
@@ -1208,11 +1220,14 @@ 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 DEVICE_ATTR(data_available, S_IRUGO,
+		iio_dma_show_data_available, NULL);
 
 static struct attribute *iio_buffer_attrs[] = {
 	&dev_attr_length.attr,
 	&dev_attr_enable.attr,
 	&dev_attr_watermark.attr,
+	&dev_attr_data_available.attr,
 };
 
 int iio_buffer_alloc_sysfs_and_mask(struct iio_dev *indio_dev)