Re: [PATCH] drm/msm/dp: add fail safe mode outside of event_mutex context

[Dmitry Baryshkov]

On Thu, 7 Apr 2022 at 19:51, Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx> wrote:
>
> There is possible circular locking dependency detected on event_mutex.
> To break this possible circular locking, this patch move setting fail
> safe mode out of event_mutex scope.

Please provide the lockdep trace here, it might help other people
trying to fix the issue or working on backports to stable kernels.

>
> Fixes: d4aca422539c ("drm/msm/dp:  always add fail-safe mode into connector mode list")
> Signed-off-by: Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx>
> ---
>  drivers/gpu/drm/msm/dp/dp_display.c |  6 ++++++
>  drivers/gpu/drm/msm/dp/dp_panel.c   | 20 ++++++++++----------
>  drivers/gpu/drm/msm/dp/dp_panel.h   |  1 +
>  3 files changed, 17 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index 178b774..a42732b 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -580,6 +580,12 @@ static int dp_hpd_plug_handle(struct dp_display_private *dp, u32 data)
>                         dp->dp_display.connector_type, state);
>         mutex_unlock(&dp->event_mutex);
>
> +       /*
> +        * add fail safe mode outside event_mutex scope
> +        * to avoid potiential circular lock with drm thread
> +        */
> +       dp_panel_add_fail_safe_mode(dp->dp_display.connector);
> +
>         /* uevent will complete connection part */
>         return 0;
>  };
> diff --git a/drivers/gpu/drm/msm/dp/dp_panel.c b/drivers/gpu/drm/msm/dp/dp_panel.c
> index f141872..26c3653 100644
> --- a/drivers/gpu/drm/msm/dp/dp_panel.c
> +++ b/drivers/gpu/drm/msm/dp/dp_panel.c
> @@ -151,6 +151,15 @@ static int dp_panel_update_modes(struct drm_connector *connector,
>         return rc;
>  }
>
> +void dp_panel_add_fail_safe_mode(struct drm_connector *connector)
> +{
> +       /* fail safe edid */
> +       mutex_lock(&connector->dev->mode_config.mutex);
> +       if (drm_add_modes_noedid(connector, 640, 480))
> +               drm_set_preferred_mode(connector, 640, 480);
> +       mutex_unlock(&connector->dev->mode_config.mutex);
> +}
> +
>  int dp_panel_read_sink_caps(struct dp_panel *dp_panel,
>         struct drm_connector *connector)
>  {
> @@ -207,16 +216,7 @@ int dp_panel_read_sink_caps(struct dp_panel *dp_panel,
>                         goto end;
>                 }
>
> -               /* fail safe edid */
> -               mutex_lock(&connector->dev->mode_config.mutex);
> -               if (drm_add_modes_noedid(connector, 640, 480))
> -                       drm_set_preferred_mode(connector, 640, 480);
> -               mutex_unlock(&connector->dev->mode_config.mutex);
> -       } else {
> -               /* always add fail-safe mode as backup mode */
> -               mutex_lock(&connector->dev->mode_config.mutex);
> -               drm_add_modes_noedid(connector, 640, 480);
> -               mutex_unlock(&connector->dev->mode_config.mutex);
> +               dp_panel_add_fail_safe_mode(connector);
>         }
>
>         if (panel->aux_cfg_update_done) {
> diff --git a/drivers/gpu/drm/msm/dp/dp_panel.h b/drivers/gpu/drm/msm/dp/dp_panel.h
> index 9023e5b..99739ea 100644
> --- a/drivers/gpu/drm/msm/dp/dp_panel.h
> +++ b/drivers/gpu/drm/msm/dp/dp_panel.h
> @@ -59,6 +59,7 @@ int dp_panel_init_panel_info(struct dp_panel *dp_panel);
>  int dp_panel_deinit(struct dp_panel *dp_panel);
>  int dp_panel_timing_cfg(struct dp_panel *dp_panel);
>  void dp_panel_dump_regs(struct dp_panel *dp_panel);
> +void dp_panel_add_fail_safe_mode(struct drm_connector *connector);
>  int dp_panel_read_sink_caps(struct dp_panel *dp_panel,
>                 struct drm_connector *connector);
>  u32 dp_panel_get_mode_bpp(struct dp_panel *dp_panel, u32 mode_max_bpp,
> --
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>


-- 
With best wishes
Dmitry