diff options
| author | Daniel Vetter <daniel.vetter@ffwll.ch> | 2016-01-22 08:53:45 +0100 |
|---|---|---|
| committer | Daniel Vetter <daniel.vetter@ffwll.ch> | 2016-01-25 08:32:22 +0100 |
| commit | 40f8cf4b5cb606c593be25cbf71c34d505815b90 (patch) | |
| tree | 37a949d2724ac21af2794fe5d0e035bc212d8be5 /drivers/gpu/drm/drm_fb_helper.c | |
| parent | 70c0616d5a8458ae9148d74309cede07ba2f5164 (diff) | |
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>
Diffstat (limited to 'drivers/gpu/drm/drm_fb_helper.c')
| -rw-r--r-- | drivers/gpu/drm/drm_fb_helper.c | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c index 1e103c4c6ee0..76a364e62081 100644 --- 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. */ |