1 files changed, 48 insertions, 2 deletions
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 7d6deaa91281..9f4241f0dd9c 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -647,13 +647,59 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
 #define DRM_CORE_IOCTL_COUNT	ARRAY_SIZE( drm_ioctls )
 
 /**
+ * DOC: driver specific ioctls
+ *
+ * First things first, driver private IOCTLs should only be needed for drivers
+ * supporting rendering. Kernel modesetting is all standardized, and extended
+ * through properties. There are a few exceptions in some existing drivers,
+ * which define IOCTL for use by the display DRM master, but they all predate
+ * properties.
+ *
+ * Now if you do have a render driver you always have to support it through
+ * driver private properties. There's a few steps needed to wire all the things
+ * up.
+ *
+ * First you need to define the structure for your IOCTL in your driver private
+ * UAPI header in ``include/uapi/drm/my_driver_drm.h``::
+ *
+ *     struct my_driver_operation {
+ *             u32 some_thing;
+ *             u32 another_thing;
+ *     };
+ *
+ * Please make sure that you follow all the best practices from
+ * ``Documentation/ioctl/botching-up-ioctls.txt``. Note that drm_ioctl()
+ * automatically zero-extends structures, hence make sure you can add more stuff
+ * at the end, i.e. don't put a variable sized array there.
+ *
+ * Then you need to define your IOCTL number, using one of DRM_IO(), DRM_IOR(),
+ * DRM_IOW() or DRM_IOWR(). It must start with the DRM_IOCTL\_ prefix::
+ *
+ *     ##define DRM_IOCTL_MY_DRIVER_OPERATION \
+ *         DRM_IOW(DRM_COMMAND_BASE, struct my_driver_operation)
+ * 
+ * DRM driver private IOCTL must be in the range from DRM_COMMAND_BASE to
+ * DRM_COMMAND_END. Finally you need an array of &struct drm_ioctl_desc to wire
+ * up the handlers and set the access rights:
+ *
+ *     static const struct drm_ioctl_desc my_driver_ioctls[] = {
+ *         DRM_IOCTL_DEF_DRV(MY_DRIVER_OPERATION, my_driver_operation,
+ *                 DRM_AUTH|DRM_RENDER_ALLOW),
+ *     };
+ *
+ * And then assign this to the &drm_driver.ioctls field in your driver
+ * structure.
+ */
+
+/**
  * drm_ioctl - ioctl callback implementation for DRM drivers
  * @filp: file this ioctl is called on
  * @cmd: ioctl cmd number
  * @arg: user argument
  *
- * Looks up the ioctl function in the ::ioctls table, checking for root
- * previleges if so required, and dispatches to the respective function.
+ * Looks up the ioctl function in the DRM core and the driver dispatch table,
+ * stored in &drm_driver.ioctls. It checks for necessary permission by calling
+ * drm_ioctl_permit(), and dispatches to the respective function.
  *
  * Returns:
  * Zero on success, negative error code on failure.