Re: [PATCH] drm/atomic: clear the state of plane which is unused.

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Mar 16, 2026 at 03:55:07PM -0700, Austin Hu wrote:
> Some libdrm user mode apps (including compositor) disable unused
> plane with its previously binded CRTC or frame buffer object, but
> may not reset its properties like src/dst coordinates, so that some
> values of the unused plane state would be sticky. And even with
> some Display Engine IP like Intel, a plane disabled from user mode
> in current frame may be linked with another plane (e.g. to DMA Y or
> UV semi-planar buffer) in KMD driver, so that its sticky UAPI
> settings mismatch with the ones programmed to registers.
> 
> So clear the unused plane state during each atomic commit, to make
> sure each plane state is up to date. This change might be
> unnecessary if user mode already clears plane property, but keep it
> just in case.

The kernel isn't allowed to magically change the value of
properties set by userspace, apart from a few special cases.

No idea what you're actually trying to fix here, but sounds
like your userspace might be broken and needs to be fixed.

> 
> Signed-off-by: Austin Hu <austin.hu@intel.com>
> ---
>  drivers/gpu/drm/drm_atomic.c | 62 ++++++++++++++++++++++++++++++++++--
>  1 file changed, 59 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index dd9f27cfe991..0ce57e540a86 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -690,6 +690,59 @@ plane_switching_crtc(const struct drm_plane_state *old_plane_state,
>  	return true;
>  }
>  
> +/**
> + * drm_atomic_plane_state_reset - reset plane state
> + * @plane: plane to reset
> + * @plane_state: plane state to reset
> + *
> + * This function resets the plane state to its default values, after
> + * the plane is disabled or when the plane state is no longer valid.
> + */
> +static int drm_atomic_plane_state_reset(struct drm_plane *plane,
> +				      struct drm_plane_state *plane_state)
> +{
> +	int ret;
> +
> +	ret = drm_atomic_set_crtc_for_plane(plane_state, NULL);
> +	if (ret != 0)
> +		return ret;
> +
> +	drm_atomic_set_fb_for_plane(plane_state, NULL);
> +
> +	plane_state->crtc_x = 0;
> +	plane_state->crtc_y = 0;
> +	plane_state->crtc_w = 0;
> +	plane_state->crtc_h = 0;
> +	plane_state->src_x = 0;
> +	plane_state->src_y = 0;
> +	plane_state->src_w = 0;
> +	plane_state->src_h = 0;
> +
> +	plane_state->alpha = 0;
> +	plane_state->pixel_blend_mode = 0;
> +	plane_state->rotation = 0;
> +	plane_state->zpos = 0;
> +	plane_state->color_encoding = 0;
> +	plane_state->color_range = 0;
> +
> +	drm_atomic_set_colorop_for_plane(plane_state, NULL);
> +
> +	bool replaced = false;
> +
> +	ret = drm_property_replace_blob_from_id(
> +		plane->dev, &plane_state->fb_damage_clips, 0, -1, -1,
> +		sizeof(struct drm_mode_rect), &replaced);
> +	if (ret)
> +		return ret;
> +
> +	plane_state->scaling_filter = 0;
> +	plane_state->hotspot_x = 0;
> +	plane_state->hotspot_y = 0;
> +	plane_state->visible = false;
> +
> +	return 0;
> +}
> +
>  /**
>   * drm_atomic_plane_check - check plane state
>   * @old_plane_state: old plane state to check
> @@ -701,7 +754,7 @@ plane_switching_crtc(const struct drm_plane_state *old_plane_state,
>   * Zero on success, error code on failure
>   */
>  static int drm_atomic_plane_check(const struct drm_plane_state *old_plane_state,
> -				  const struct drm_plane_state *new_plane_state)
> +				  struct drm_plane_state *new_plane_state)
>  {
>  	struct drm_plane *plane = new_plane_state->plane;
>  	struct drm_crtc *crtc = new_plane_state->crtc;
> @@ -721,9 +774,12 @@ static int drm_atomic_plane_check(const struct drm_plane_state *old_plane_state,
>  		return -EINVAL;
>  	}
>  
> -	/* if disabled, we don't care about the rest of the state: */
> +	/*
> +	 * if disabled, we don't care about the rest of the state:
> +	 * but clear the plane state.
> +	 */
>  	if (!crtc)
> -		return 0;
> +		return drm_atomic_plane_state_reset(plane, new_plane_state);
>  
>  	/* Check whether this plane is usable on this CRTC */
>  	if (!(plane->possible_crtcs & drm_crtc_mask(crtc))) {
> -- 
> 2.34.1

-- 
Ville Syrjälä
Intel