drm/fb-helper: Clarify use of last_close and output_poll_changed

Clarify documentation in the use of struct drm_driver.last_close and
struct drm_mode_config_funcs.output_poll_changed. Those callbacks should
not be said for fbdev implementations on top of struct drm_client_funcs.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221103151446.2638-24-tzimmermann@suse.de

drivers/gpu/drm/drm_fb_helper.c

@@ -89,11 +89,13 @@ static DEFINE_MUTEX(kernel_fb_helper_lock);
  * It will automatically set up deferred I/O if the driver requires a shadow
  * buffer.
  *
- * At runtime drivers should restore the fbdev console by using
+ * Existing fbdev implementations should restore the fbdev console by using
  * drm_fb_helper_lastclose() as their &drm_driver.lastclose callback.
  * They should also notify the fb helper code from updates to the output
  * configuration by using drm_fb_helper_output_poll_changed() as their
- * &drm_mode_config_funcs.output_poll_changed callback.
+ * &drm_mode_config_funcs.output_poll_changed callback. New implementations
+ * of fbdev should be build on top of struct &drm_client_funcs, which handles
+ * this automatically. Setting the old callbacks should be avoided.
  *
  * For suspend/resume consider using drm_mode_config_helper_suspend() and
  * drm_mode_config_helper_resume() which takes care of fbdev as well.