[RFC PATCH 2/4] media: Add Media Device Allocator API documentation
From: Shuah Khan
Date: Sat Mar 26 2016 - 00:39:39 EST
Add Media Device Allocator API documentation.
Signed-off-by: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
include/media/media-dev-allocator.h | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)
diff --git a/include/media/media-dev-allocator.h b/include/media/media-dev-allocator.h
index 2932c90..174840c 100644
@@ -20,6 +20,38 @@
+ * DOC: Media Controller Device Allocator API
+ * There are known problems with media device life time management. When media
+ * device is released while an media ioctl is in progress, ioctls fail with
+ * use-after-free errors and kernel hangs in some cases.
+ * Media Device can be in any the following states:
+ * - Allocated
+ * - Registered (could be tied to more than one driver)
+ * - Unregistered, not in use (media device file is not open)
+ * - Unregistered, in use (media device file is not open)
+ * - Released
+ * When media device belongs to more than one driver, registrations should be
+ * refcounted to avoid unregistering when one of the drivers does unregister.
+ * A refcount field in the struct media_device covers this case. Unregister on
+ * a Media Allocator media device is a kref_put() call. The media device should
+ * be unregistered only when the last unregister occurs.
+ * When a media device is in use when it is unregistered, it should not be
+ * released until the application exits when it detects the unregistered
+ * status. Media device that is in use when it is unregistered is moved to
+ * to_delete_list. When the last unregister occurs, media device is unregistered
+ * and becomes an unregistered, still allocated device. Unregister marks the
+ * device to be deleted.
+ * When media device belongs to more than one driver, as both drivers could be
+ * unbound/bound, driver should not end up getting stale media device that is
+ * on its way out. Moving the unregistered media device to to_delete_list helps
+ * this case as well.
* media_device_get() - Allocate and return global media device