Hi Daniel,
Thanks for the tip.
I just enabled fb.lockless_register_fb=1 and still cannot see any debug
messages after the console_lock... :(
Regards,
C.Palminha
On 22-01-2016 07:53, Daniel Vetter wrote:
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@xxxxxxxxxxxx>
Cc: Xinliang Liu <xinliang.liu@xxxxxxxxxx>
Cc: laurent.pinchart@xxxxxxxxxxxxxxxx
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
---
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.
*
+ * HANG DEBUGGING:
+ *
+ * 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.
+ *
* RETURNS:
* Zero if everything went ok, nonzero otherwise.
*/
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