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Commit 40f8cf4b authored by Daniel Vetter's avatar Daniel Vetter
Browse files

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: default avatarLaurent 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: default avatarDaniel Vetter <daniel.vetter@intel.com>
parent 70c0616d

drivers/gpu/drm/drm_fb_helper.c

+21 −0
Original line number Original line Diff line number Diff line
@@ -2091,6 +2091,27 @@ static void drm_setup_crtcs(struct drm_fb_helper *fb_helper) 
 * drm_fb_helper_fill_fix() are provided as helpers to setup simple default

 * drm_fb_helper_fill_fix() are provided as helpers to setup simple default

 * values for the fbdev info structure.

 * 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:

 * RETURNS:

 * Zero if everything went ok, nonzero otherwise.

 * Zero if everything went ok, nonzero otherwise.

 */

 */