drm/fbdev-helper: Explain how to debug console_lock fun

Every new KMS driver writer seems to run into this and wonder how
exactly drm_fb_helper_initial_config can die doing nothing at all.
Set up some big warnings signs around this newbie trap to avoid future
frustration and wasting everyone's time.

v2: Edits from Laurent.

Cc: Carlos Palminha <CARLOS.PALMINHA@synopsys.com>
Cc: Xinliang Liu <xinliang.liu@linaro.org>
Cc: laurent.pinchart@ideasonboard.com
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453449225-10954-1-git-send-email-daniel.vetter@ffwll.ch
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_fb_helper.c

@@ -2091,6 +2091,27 @@ out:
  * drm_fb_helper_fill_fix() are provided as helpers to setup simple default
  * values for the fbdev info structure.
  *
+ * HANG DEBUGGING:
+ *
+ * When you have fbcon support built-in or already loaded, this function will do
+ * a full modeset to setup the fbdev console. Due to locking misdesign in the
+ * VT/fbdev subsystem that entire modeset sequence has to be done while holding
+ * console_lock. Until console_unlock is called no dmesg lines will be sent out
+ * to consoles, not even serial console. This means when your driver crashes,
+ * you will see absolutely nothing else but a system stuck in this function,
+ * with no further output. Any kind of printk() you place within your own driver
+ * or in the drm core modeset code will also never show up.
+ *
+ * Standard debug practice is to run the fbcon setup without taking the
+ * console_lock as a hack, to be able to see backtraces and crashes on the
+ * serial line. This can be done by setting the fb.lockless_register_fb=1 kernel
+ * cmdline option.
+ *
+ * The other option is to just disable fbdev emulation since very likely the
+ * first modest from userspace will crash in the same way, and is even easier to
+ * debug. This can be done by setting the drm_kms_helper.fbdev_emulation=0
+ * kernel cmdline option.
+ *
  * RETURNS:
  * Zero if everything went ok, nonzero otherwise.
  */