[PATCH 2/2] drm: document DRM_IOCTL_GEM_CLOSE

[Simon Ser]

This is a bit tricky, because of the ref'counting considerations.
See also [1] for more discussion about this topic. Since this is
kernel docs, I've decided to elaborate a bit less on the user-space
details.

[1]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/110

Signed-off-by: Simon Ser <contact at emersion.fr>
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: Pekka Paalanen <ppaalanen at gmail.com>
Cc: Daniel Stone <daniel at fooishbar.org>
---
 include/uapi/drm/drm.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
index 54b2313c8332..4829f1fa9570 100644
--- a/include/uapi/drm/drm.h
+++ b/include/uapi/drm/drm.h
@@ -972,6 +972,19 @@ extern "C" {
 #define DRM_IOCTL_GET_STATS             DRM_IOR( 0x06, struct drm_stats)
 #define DRM_IOCTL_SET_VERSION		DRM_IOWR(0x07, struct drm_set_version)
 #define DRM_IOCTL_MODESET_CTL           DRM_IOW(0x08, struct drm_modeset_ctl)
+/**
+ * DRM_IOCTL_GEM_CLOSE - Close a GEM handle.
+ *
+ * GEM handles are not reference-counted by the kernel. User-space is
+ * responsible for managing their lifetime. For example, if user-space imports
+ * the same memory object twice on the same DRM file description, the same GEM
+ * handle is returned by both imports, and user-space needs to ensure
+ * &DRM_IOCTL_GEM_CLOSE is performed once only. The same situation can happen
+ * when a memory object is allocated, then exported and imported again on the
+ * same DRM file description. The &DRM_IOCTL_MODE_GETFB2 IOCTL is an exception
+ * and always returns fresh new GEM handles even if an existing GEM handle
+ * already refers to the same memory object before the IOCTL is performed.
+ */
 #define DRM_IOCTL_GEM_CLOSE		DRM_IOW (0x09, struct drm_gem_close)
 #define DRM_IOCTL_GEM_FLINK		DRM_IOWR(0x0a, struct drm_gem_flink)
 #define DRM_IOCTL_GEM_OPEN		DRM_IOWR(0x0b, struct drm_gem_open)
-- 
2.39.1