Re: [PATCH] drm/mediatek: Add wait_event_timeout when disabling plane

["CK Hu (胡俊光)" <ck.hu@mediatek.com>]

On Thu, 2025-05-22 at 16:34 +0800, Jason-JH Lin wrote:
> Our hardware registers are set through GCE, not by the CPU.
> DRM might assume the hardware is disabled immediately after calling
> atomic_disable() of drm_plane, but it is only truly disabled after the
> GCE IRQ is triggered.
> 
> Additionally, the cursor plane in DRM uses async_commit, so DRM will
> not wait for vblank and will free the buffer immediately after calling
> atomic_disable().
> 
> To prevent the framebuffer from being freed before the layer disable
> settings are configured into the hardware, which can cause an IOMMU
> fault error, a wait_event_timeout has been added to wait for the
> ddp_cmdq_cb() callback,indicating that the GCE IRQ has been triggered.
> 
> Fixes: 119f5173628a ("drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.")
> Signed-off-by: Jason-JH Lin <jason-jh.lin@mediatek.com>
> ---
>  drivers/gpu/drm/mediatek/mtk_crtc.c  | 30 ++++++++++++++++++++++++++++
>  drivers/gpu/drm/mediatek/mtk_crtc.h  |  1 +
>  drivers/gpu/drm/mediatek/mtk_plane.c |  5 +++++
>  3 files changed, 36 insertions(+)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_crtc.c b/drivers/gpu/drm/mediatek/mtk_crtc.c
> index 8f6fba4217ec..944a3d1e5ec9 100644
> --- a/drivers/gpu/drm/mediatek/mtk_crtc.c
> +++ b/drivers/gpu/drm/mediatek/mtk_crtc.c
> @@ -719,6 +719,36 @@ int mtk_crtc_plane_check(struct drm_crtc *crtc, struct drm_plane *plane,
>  	return 0;
>  }
>  
> +void mtk_crtc_plane_disable(struct drm_crtc *crtc, struct drm_plane *plane)
> +{
> +	struct mtk_crtc *mtk_crtc = to_mtk_crtc(crtc);
> +	struct mtk_plane_state *plane_state = to_mtk_plane_state(plane->state);
> +	int i;
> +
> +	if (!mtk_crtc->enabled)
> +		return;
> +
> +	/* set pending plane state to disabled */
> +	for (i = 0; i < mtk_crtc->layer_nr; i++) {
> +		struct drm_plane *mtk_plane = &mtk_crtc->planes[i];
> +		struct mtk_plane_state *mtk_plane_state = to_mtk_plane_state(mtk_plane->state);
> +
> +		if (mtk_plane->index == plane->index) {
> +			memcpy(mtk_plane_state, plane_state, sizeof(*plane_state));

In commit message, you mention GCE flow has problem.
This also modify non-GCE flow.
If non-GCE flow does not need this, move this to GCE flow.

> +			break;
> +		}
> +	}
> +	mtk_crtc_update_config(mtk_crtc, false);
> +
> +#if IS_REACHABLE(CONFIG_MTK_CMDQ)
> +	/* wait for planes to be disabled by cmdq */
> +	if (mtk_crtc->cmdq_client.chan)
> +		wait_event_timeout(mtk_crtc->cb_blocking_queue,
> +				   mtk_crtc->cmdq_vblank_cnt == 0,

Check 'mtk_crtc->cmdq_vblank_cnt == 0' may be not good.
If a video is playing and mtk_crtc_update_config() would be call every frame,
mtk_crtc->cmdq_vblank_cnt may not be zero and cursor would be block until timeout.

Regards,
CK

> +				   msecs_to_jiffies(500));
> +#endif
> +}
> +
>  void mtk_crtc_async_update(struct drm_crtc *crtc, struct drm_plane *plane,
>  			   struct drm_atomic_state *state)
>  {
> diff --git a/drivers/gpu/drm/mediatek/mtk_crtc.h b/drivers/gpu/drm/mediatek/mtk_crtc.h
> index 388e900b6f4d..828f109b83e7 100644
> --- a/drivers/gpu/drm/mediatek/mtk_crtc.h
> +++ b/drivers/gpu/drm/mediatek/mtk_crtc.h
> @@ -21,6 +21,7 @@ int mtk_crtc_create(struct drm_device *drm_dev, const unsigned int *path,
>  		    unsigned int num_conn_routes);
>  int mtk_crtc_plane_check(struct drm_crtc *crtc, struct drm_plane *plane,
>  			 struct mtk_plane_state *state);
> +void mtk_crtc_plane_disable(struct drm_crtc *crtc, struct drm_plane *plane);
>  void mtk_crtc_async_update(struct drm_crtc *crtc, struct drm_plane *plane,
>  			   struct drm_atomic_state *plane_state);
>  struct device *mtk_crtc_dma_dev_get(struct drm_crtc *crtc);
> diff --git a/drivers/gpu/drm/mediatek/mtk_plane.c b/drivers/gpu/drm/mediatek/mtk_plane.c
> index 655106bbb76d..59edbe26f01e 100644
> --- a/drivers/gpu/drm/mediatek/mtk_plane.c
> +++ b/drivers/gpu/drm/mediatek/mtk_plane.c
> @@ -285,9 +285,14 @@ static void mtk_plane_atomic_disable(struct drm_plane *plane,
>  	struct drm_plane_state *new_state = drm_atomic_get_new_plane_state(state,
>  									   plane);
>  	struct mtk_plane_state *mtk_plane_state = to_mtk_plane_state(new_state);
> +	struct drm_plane_state *old_state = drm_atomic_get_old_plane_state(state,
> +									   plane);
> +
>  	mtk_plane_state->pending.enable = false;
>  	wmb(); /* Make sure the above parameter is set before update */
>  	mtk_plane_state->pending.dirty = true;
> +
> +	mtk_crtc_plane_disable(old_state->crtc, plane);
>  }
>  
>  static void mtk_plane_atomic_update(struct drm_plane *plane,