[PATCH] drm/drm_vblank: use drm_warn_once() to warn undefined mode timing

[Daniel Vetter]

On Fri, Oct 16, 2020 at 9:13 AM Shawn Guo <shawn.guo at linaro.org> wrote:
>
> Commit 5caa0feafcc6 ("drm/vblank: Lock down vblank->hwmode more") added
> WARN_ON_ONCE() for atomic drivers to warn the case that vsync is enabled
> before a mode has been set on CRTC.  This happens sometimes during the
> initial mode setting of a CRTC.  It also happens on Android running HWC2
> backed with drm_hwcomposer, where HWC2::SetVsyncEnabled could be called
> before the atomic mode setting on CRTC happens.
>
> In this case, there is nothing really bad to happen as kernel function
> returns as no-op.  So using WARN() version might be overkilled,
> considering some user space crash reporting services may treat kernel
> WARNINGS as crashes.  Let's drop WARN_ON_ONCE() and change drm_dbg_core()
> to drm_warn_once() for warning undefined mode timing.

This indicates a bug in your driver. Please fix it there, not by
shutting up the core code complaining about that. Either you're
getting vblank timestamps when the vblank isn't set up yet
(drm_crtc_vblank_on/off) or there's some other race going on in your
driver code resulting in this.
-Daniel

> Signed-off-by: Shawn Guo <shawn.guo at linaro.org>
> ---
>  drivers/gpu/drm/drm_vblank.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index b18e1efbbae1..5f7c4a923e8a 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -717,9 +717,9 @@ drm_crtc_vblank_helper_get_vblank_timestamp_internal(
>          * Happens during initial modesetting of a crtc.
>          */
>         if (mode->crtc_clock == 0) {
> -               drm_dbg_core(dev, "crtc %u: Noop due to uninitialized mode.\n",
> -                            pipe);
> -               drm_WARN_ON_ONCE(dev, drm_drv_uses_atomic_modeset(dev));
> +               drm_warn_once(dev, "crtc %u: Noop due to uninitialized mode.\n",
> +                             pipe);
> +
>                 return false;
>         }
>
> --
> 2.17.1
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch