iio: core: implement iio_device_{claim|release}_direct_mode()

It is often the case that the driver wants to be sure a device stays
in direct mode while it is executing a task or series of tasks.  To
accomplish this today, the driver performs this sequence: 1) take the
device state lock, 2) verify it is not in a buffered mode, 3) execute
some tasks, and 4) release that lock.

This patch introduces a pair of helper functions that simplify these
steps and make it more semantically expressive.

iio_device_claim_direct_mode()
        If the device is not in any buffered mode it is guaranteed
        to stay that way until iio_release_direct_mode() is called.

iio_device_release_direct_mode()
        Release the claim. Device is no longer guaranteed to stay
        in direct mode.
Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-core.c

@@ -25,6 +25,7 @@
 #include <linux/slab.h>
 #include <linux/anon_inodes.h>
 #include <linux/debugfs.h>
+#include <linux/mutex.h>
 #include <linux/iio/iio.h>
 #include "iio_core.h"
 #include "iio_core_trigger.h"
@@ -1375,6 +1376,44 @@ void devm_iio_device_unregister(struct device *dev, struct iio_dev *indio_dev)
 }
 EXPORT_SYMBOL_GPL(devm_iio_device_unregister);
 
+/**
+ * iio_device_claim_direct_mode - Keep device in direct mode
+ * @indio_dev:	the iio_dev associated with the device
+ *
+ * If the device is in direct mode it is guaranteed to stay
+ * that way until iio_device_release_direct_mode() is called.
+ *
+ * Use with iio_device_release_direct_mode()
+ *
+ * Returns: 0 on success, -EBUSY on failure
+ */
+int iio_device_claim_direct_mode(struct iio_dev *indio_dev)
+{
+	mutex_lock(&indio_dev->mlock);
+
+	if (iio_buffer_enabled(indio_dev)) {
+		mutex_unlock(&indio_dev->mlock);
+		return -EBUSY;
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(iio_device_claim_direct_mode);
+
+/**
+ * iio_device_release_direct_mode - releases claim on direct mode
+ * @indio_dev:	the iio_dev associated with the device
+ *
+ * Release the claim. Device is no longer guaranteed to stay
+ * in direct mode.
+ *
+ * Use with iio_device_claim_direct_mode()
+ */
+void iio_device_release_direct_mode(struct iio_dev *indio_dev)
+{
+	mutex_unlock(&indio_dev->mlock);
+}
+EXPORT_SYMBOL_GPL(iio_device_release_direct_mode);
+
 subsys_initcall(iio_init);
 module_exit(iio_exit);
 

include/linux/iio/iio.h

@@ -527,6 +527,8 @@ void iio_device_unregister(struct iio_dev *indio_dev);
 int devm_iio_device_register(struct device *dev, struct iio_dev *indio_dev);
 void devm_iio_device_unregister(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);
 
 extern struct bus_type iio_bus_type;