[Intel-gfx] [PATCH] drm/fbdev-helper: Explain how to debug console_lock fun

[Carlos Palminha]

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 at synopsys.com>
> Cc: Xinliang Liu <xinliang.liu at linaro.org>
> Cc: laurent.pinchart at ideasonboard.com
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> ---
>   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.
>    */
>