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. */ -- 2.7.0.rc3 _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
