drm: Include the encoder itself in possible_clones

The docs say possible_clones should always include the encoder itself.
Since most drivers don't want to deal with the complexities of cloning
let's allow them to set possible_clones=0 and instead we'll fix that
up in the core.

We can't put this special case into drm_encoder_init() because drivers
will have to fill up possible_clones after adding all the relevant
encoders. Otherwise they wouldn't know the proper encoder indexes to
use. So we'll just do it just before registering the device.

v2: Don't set the bit if possible_clones!=0 so that the
validation (coming soon) will WARN (Thomas)
Fix up the docs to allow possible_clones==0 (Daniel)
.late_register() is too late, introduce drm_mode_config_validate()
which gets called _before_ we register the char device (Daniel)

Acked-by: Thomas Zimmermann <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Signed-off-by: Ville Syrjälä <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/drm_crtc_internal.h

···
 /* drm_mode_config.c */
 int drm_modeset_register_all(struct drm_device *dev);
 void drm_modeset_unregister_all(struct drm_device *dev);
+void drm_mode_config_validate(struct drm_device *dev);
 
 /* drm_modes.c */
 const char *drm_get_mode_status_name(enum drm_mode_status status);

drivers/gpu/drm/drm_drv.c

···
 	struct drm_driver *driver = dev->driver;
 	int ret;
 
+	if (!driver->load)
+		drm_mode_config_validate(dev);
+
 	if (drm_dev_needs_global_mutex(dev))
 		mutex_lock(&drm_global_mutex);
 

drivers/gpu/drm/drm_mode_config.c

···
 	drm_modeset_lock_fini(&dev->mode_config.connection_mutex);
 }
 EXPORT_SYMBOL(drm_mode_config_cleanup);
+
+/*
+ * For some reason we want the encoder itself included in
+ * possible_clones. Make life easy for drivers by allowing them
+ * to leave possible_clones unset if no cloning is possible.
+ */
+static void fixup_encoder_possible_clones(struct drm_encoder *encoder)
+{
+	if (encoder->possible_clones == 0)
+		encoder->possible_clones = drm_encoder_mask(encoder);
+}
+
+void drm_mode_config_validate(struct drm_device *dev)
+{
+	struct drm_encoder *encoder;
+
+	drm_for_each_encoder(encoder, dev)
+		fixup_encoder_possible_clones(encoder);
+}

include/drm/drm_encoder.h

···
 	 * encoders can be used in a cloned configuration, they both should have
 	 * each another bits set.
 	 *
-	 * In reality almost every driver gets this wrong.
+	 * As an exception to the above rule if the driver doesn't implement
+	 * any cloning it can leave @possible_clones set to 0. The core will
+	 * automagically fix this up by setting the bit for the encoder itself.
 	 *
 	 * Note that since encoder objects can't be hotplugged the assigned indices
 	 * are stable and hence known before registering all objects.