Merge remote-tracking branch 'sound/for-next'

[karo-tx-linux.git] / drivers / gpu / drm / drm_fb_helper.c

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 1e103c4c6ee056d5011178bbbe8675d490f94a8f..76a364e620811d0f828eab3a1b1c939f95b6c9ba 100644 (file)
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/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.
  */