From: Daniel Vetter Date: Fri, 22 Jan 2016 07:53:45 +0000 (+0100) Subject: drm/fbdev-helper: Explain how to debug console_lock fun X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=40f8cf4b5cb606c593be25cbf71c34d505815b90;p=GitHub%2Fmoto-9609%2Fandroid_kernel_motorola_exynos9610.git 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 Cc: Xinliang Liu Cc: laurent.pinchart@ideasonboard.com Reviewed-by: Laurent Pinchart Link: http://patchwork.freedesktop.org/patch/msgid/1453449225-10954-1-git-send-email-daniel.vetter@ffwll.ch Signed-off-by: Daniel Vetter --- 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. */