drm/doc: Update docs about device instance setup

->load is deprecated, bus functions are deprecated and everyone
should use drm_dev_alloc&register.

So update the .tmpl (and pull a bunch of the overview docs into the
sourcecode to increase chances that it'll stay in sync in the future)
and add notes to functions which are deprecated. I didn't bother to
clean up and document the unload sequence similarly since that one is
still a bit a mess: drm_dev_unregister does way too much,
drm_unplug_dev does what _unregister should be doing but then has the
complication of promising something it doesn't actually do (it doesn't
unplug existing open fds for instance, only prevents new ones).

Motivated since I don't want to hunt every new driver for usage of
drm_platform_init any more ;-)

v2: Reword the deprecation note for ->load a bit, using Laurent's
suggestion as an example (but making the wording a bit stronger even).
Fix spelling in commit message.

v3: More spelling fixes from Laurent.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Acked-by: David Herrmann <dh.herrmann@gmail.com> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_drv.c

@@ -396,16 +396,52 @@ void drm_minor_release(struct drm_minor *minor)
 	drm_dev_unref(minor->dev);
 }
 
+/**
+ * DOC: driver instance overview
+ *
+ * A device instance for a drm driver is represented by struct &drm_device. This
+ * is allocated with drm_dev_alloc(), usually from bus-specific ->probe()
+ * callbacks implemented by the driver. The driver then needs to initialize all
+ * the various subsystems for the drm device like memory management, vblank
+ * handling, modesetting support and intial output configuration plus obviously
+ * initialize all the corresponding hardware bits. An important part of this is
+ * also calling drm_dev_set_unique() to set the userspace-visible unique name of
+ * this device instance. Finally when everything is up and running and ready for
+ * userspace the device instance can be published using drm_dev_register().
+ *
+ * There is also deprecated support for initalizing device instances using
+ * bus-specific helpers and the ->load() callback. But due to
+ * backwards-compatibility needs the device instance have to be published too
+ * early, which requires unpretty global locking to make safe and is therefore
+ * only support for existing drivers not yet converted to the new scheme.
+ *
+ * When cleaning up a device instance everything needs to be done in reverse:
+ * First unpublish the device instance with drm_dev_unregister(). Then clean up
+ * any other resources allocated at device initialization and drop the driver's
+ * reference to &drm_device using drm_dev_unref().
+ *
+ * Note that the lifetime rules for &drm_device instance has still a lot of
+ * historical baggage. Hence use the reference counting provided by
+ * drm_dev_ref() and drm_dev_unref() only carefully.
+ *
+ * Also note that embedding of &drm_device is currently not (yet) supported (but
+ * it would be easy to add). Drivers can store driver-private data in the
+ * dev_priv field of &drm_device.
+ */
+
 /**
  * drm_put_dev - Unregister and release a DRM device
  * @dev: DRM device
  *
  * Called at module unload time or when a PCI device is unplugged.
  *
- * Use of this function is discouraged. It will eventually go away completely.
- * Please use drm_dev_unregister() and drm_dev_unref() explicitly instead.
- *
  * Cleans up all DRM device, calling drm_lastclose().
+ *
+ * Note: Use of this function is deprecated. It will eventually go away
+ * completely.  Please use drm_dev_unregister() and drm_dev_unref() explicitly
+ * instead to make sure that the device isn't userspace accessible any more
+ * while teardown is in progress, ensuring that userspace can't access an
+ * inconsistent state.
  */
 void drm_put_dev(struct drm_device *dev)
 {
@@ -518,7 +554,9 @@ static void drm_fs_inode_free(struct inode *inode)
  *
  * Allocate and initialize a new DRM device. No device registration is done.
  * Call drm_dev_register() to advertice the device to user space and register it
- * with other core subsystems.
+ * with other core subsystems. This should be done last in the device
+ * initialization sequence to make sure userspace can't access an inconsistent
+ * state.
  *
  * The initial ref-count of the object is 1. Use drm_dev_ref() and
  * drm_dev_unref() to take and drop further ref-counts.
@@ -673,6 +711,12 @@ EXPORT_SYMBOL(drm_dev_unref);
  *
  * Never call this twice on any device!
  *
+ * NOTE: To ensure backward compatibility with existing drivers method this
+ * function calls the ->load() method after registering the device nodes,
+ * creating race conditions. Usage of the ->load() methods is therefore
+ * deprecated, drivers must perform all initialization before calling
+ * drm_dev_register().
+ *
  * RETURNS:
  * 0 on success, negative error code on failure.
  */
@@ -720,6 +764,9 @@ EXPORT_SYMBOL(drm_dev_register);
  * Unregister the DRM device from the system. This does the reverse of
  * drm_dev_register() but does not deallocate the device. The caller must call
  * drm_dev_unref() to drop their final reference.
+ *
+ * This should be called first in the device teardown code to make sure
+ * userspace can't access the device instance any more.
  */
 void drm_dev_unregister(struct drm_device *dev)
 {