iio: core: introduce iio_device_{claim|release}_buffer_mode() APIs

These APIs are analogous to iio_device_claim_direct_mode() and
iio_device_release_direct_mode() but, as the name suggests, with the
logic flipped. While this looks odd enough, it will have at least two
users (in following changes) and it will be important to move the IIO
mlock to the private struct.
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20221012151620.1725215-2-nuno.sa@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-core.c

@@ -2083,6 +2083,44 @@ void iio_device_release_direct_mode(struct iio_dev *indio_dev)
 }
 EXPORT_SYMBOL_GPL(iio_device_release_direct_mode);
 
+/**
+ * iio_device_claim_buffer_mode - Keep device in buffer mode
+ * @indio_dev:	the iio_dev associated with the device
+ *
+ * If the device is in buffer mode it is guaranteed to stay
+ * that way until iio_device_release_buffer_mode() is called.
+ *
+ * Use with iio_device_release_buffer_mode().
+ *
+ * Returns: 0 on success, -EBUSY on failure.
+ */
+int iio_device_claim_buffer_mode(struct iio_dev *indio_dev)
+{
+	mutex_lock(&indio_dev->mlock);
+
+	if (iio_buffer_enabled(indio_dev))
+		return 0;
+
+	mutex_unlock(&indio_dev->mlock);
+	return -EBUSY;
+}
+EXPORT_SYMBOL_GPL(iio_device_claim_buffer_mode);
+
+/**
+ * iio_device_release_buffer_mode - releases claim on buffer mode
+ * @indio_dev:	the iio_dev associated with the device
+ *
+ * Release the claim. Device is no longer guaranteed to stay
+ * in buffer mode.
+ *
+ * Use with iio_device_claim_buffer_mode().
+ */
+void iio_device_release_buffer_mode(struct iio_dev *indio_dev)
+{
+	mutex_unlock(&indio_dev->mlock);
+}
+EXPORT_SYMBOL_GPL(iio_device_release_buffer_mode);
+
 /**
  * iio_device_get_current_mode() - helper function providing read-only access to
  *				   the opaque @currentmode variable

include/linux/iio/iio.h

@@ -629,6 +629,8 @@ int __devm_iio_device_register(struct device *dev, struct iio_dev *indio_dev,
 int iio_push_event(struct iio_dev *indio_dev, u64 ev_code, s64 timestamp);
 int iio_device_claim_direct_mode(struct iio_dev *indio_dev);
 void iio_device_release_direct_mode(struct iio_dev *indio_dev);
+int iio_device_claim_buffer_mode(struct iio_dev *indio_dev);
+void iio_device_release_buffer_mode(struct iio_dev *indio_dev);
 
 extern struct bus_type iio_bus_type;