[Intel-gfx] [PATCH] drm/fbdev-helper: Explain how to debug console_lock fun

Daniel Vetter daniel.vetter at ffwll.ch
Thu Jan 21 23:53:45 PST 2016

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.

Cc: Carlos Palminha <CARLOS.PALMINHA at synopsys.com>
Cc: Xinliang Liu <xinliang.liu at linaro.org>
Cc: laurent.pinchart at ideasonboard.com
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
 drivers/gpu/drm/drm_fb_helper.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 1e103c4c6ee0..9bf84b227613 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -2091,6 +2091,28 @@ out:
  * drm_fb_helper_fill_fix() are provided as helpers to setup simple default
  * values for the fbdev info structure.
+ *
+ * When you have fbcon support built-in or already loaded, this function will do
+ * a full modeset to setup the fbdev console. And due to locking misdesign in
+ * the VT/fbdev subsystem that entire modeset sequence has to be done while
+ * holding console_lock. And until console_unlock is called no dmesg lines will
+ * be sent out to consoles, not even serial console. Which means when your
+ * driver crashes, you will see absolutely nothing else but a system stuck in
+ * this function, with no further output. And 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.
+ *
  * Zero if everything went ok, nonzero otherwise.

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