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

[Laurent Pinchart]

Hi Daniel,

Thank you for the patch.

On Friday 22 January 2016 08:53:45 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

No need to start the sentence with "And", s/And due/Due/ will do.

> 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

Same here.

> will
> + * be sent out to consoles, not even serial console. Which means when your

Similarly, s/Which means when/This means that if/

> + * 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

s/And any/Any/

Apart from that,

Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

Very useful addition to the documentation, I got bitten by this more than 
once. I wonder whether we could make this information even easier to find.

> 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.
>   */

-- 
Regards,

Laurent Pinchart