Re: [PATCH] drm: Simplify fb refcounting rules around ->update_plane

[Matt Roper <matthew.d.roper@intel.com>]

On Tue, Apr 22, 2014 at 04:19:45PM +0200, Daniel Vetter wrote:
> The introduction of primary planes has apparently caused a bit of fb
> refcounting fun for people. That makes it a good time to clean up the
> arcane rules and slight differences between ->update_plane and
> ->set_config. The new rules are:
> 
> - The core holds a reference for both the new and the old fb (if
>   they're non-NULL of course) while calling into the driver through
>   either ->update_plane or ->set_config.
> 
> - Drivers may not clobber plane->fb if their callback fails. If they
>   do that, they need to store a pointer to the old fb in it again.
>   When calling into the driver plane->fb still points at the current
>   (old) framebuffer.
> 
> - The core will update the plane->fb pointer on success. Drivers can
>   do that themselves too, but aren't required to any more for the
>   primary plane.
> 
> - The core will update fb refcounts for the plane->fb pointer,
>   presuming the drivers hold up their end of the bargain.
> 
> v2: Remove now unused tmpfb (Thierry)
> 
> v3: Drop broken changes from drm_mode_setplane (Ville). Also polish
> the commit message a bit.

It looks like we might have some problems around setplane with fbid=0.
It looks like we're assuming that disabling a plane always succeeds
(which is no longer true for helper-based primary planes --- they just
return -EINVAL on disable now), so we wind up setting old_fb to the
currently scanned out framebuffer and then unref it at the end of the
function if I'm reading things correctly.  We also clobber the
plane->crtc and plane->fb pointers too when this happens.

I think the real problem here was introduced on b6ccd7b9 and I gave you
an r-b tag on that email, so that's my bad for not catching it before.
:-(


Matt

> 
> Cc: Thierry Reding <treding@nvidia.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/drm_crtc.c         |  8 ++++----
>  drivers/gpu/drm/drm_plane_helper.c | 16 ----------------
>  2 files changed, 4 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index d8b7099abece..a76000ee2a43 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -2187,16 +2187,18 @@ int drm_mode_setplane(struct drm_device *dev, void *data,
>  	}
>  
>  	drm_modeset_lock_all(dev);
> +	old_fb = plane->fb;
>  	ret = plane->funcs->update_plane(plane, crtc, fb,
>  					 plane_req->crtc_x, plane_req->crtc_y,
>  					 plane_req->crtc_w, plane_req->crtc_h,
>  					 plane_req->src_x, plane_req->src_y,
>  					 plane_req->src_w, plane_req->src_h);
>  	if (!ret) {
> -		old_fb = plane->fb;
>  		plane->crtc = crtc;
>  		plane->fb = fb;
>  		fb = NULL;
> +	} else {
> +		old_fb = NULL;
>  	}
>  	drm_modeset_unlock_all(dev);
>  
> @@ -2239,9 +2241,7 @@ int drm_mode_set_config_internal(struct drm_mode_set *set)
>  	ret = crtc->funcs->set_config(set);
>  	if (ret == 0) {
>  		crtc->primary->crtc = crtc;
> -
> -		/* crtc->fb must be updated by ->set_config, enforces this. */
> -		WARN_ON(fb != crtc->primary->fb);
> +		crtc->primary->fb = fb;
>  	}
>  
>  	list_for_each_entry(tmp, &crtc->dev->mode_config.crtc_list, head) {
> diff --git a/drivers/gpu/drm/drm_plane_helper.c b/drivers/gpu/drm/drm_plane_helper.c
> index 9540ff9f97fe..b72736d5541d 100644
> --- a/drivers/gpu/drm/drm_plane_helper.c
> +++ b/drivers/gpu/drm/drm_plane_helper.c
> @@ -124,7 +124,6 @@ int drm_primary_helper_update(struct drm_plane *plane, struct drm_crtc *crtc,
>  		.y2 = crtc->mode.vdisplay,
>  	};
>  	struct drm_connector **connector_list;
> -	struct drm_framebuffer *tmpfb;
>  	int num_connectors, ret;
>  
>  	if (!crtc->enabled) {
> @@ -177,22 +176,7 @@ int drm_primary_helper_update(struct drm_plane *plane, struct drm_crtc *crtc,
>  	set.connectors = connector_list;
>  	set.num_connectors = num_connectors;
>  
> -	/*
> -	 * set_config() adjusts crtc->primary->fb; however the DRM setplane
> -	 * code that called us expects to handle the framebuffer update and
> -	 * reference counting; save and restore the current fb before
> -	 * calling it.
> -	 *
> -	 * N.B., we call set_config() directly here rather than using
> -	 * drm_mode_set_config_internal.  We're reprogramming the same
> -	 * connectors that were already in use, so we shouldn't need the extra
> -	 * cross-CRTC fb refcounting to accomodate stealing connectors.
> -	 * drm_mode_setplane() already handles the basic refcounting for the
> -	 * framebuffers involved in this operation.
> -	 */

I think there's still some value in the part of this comment that
explains why we're choosing not to call drm_mode_set_config_internal().

> -	tmpfb = plane->fb;
>  	ret = crtc->funcs->set_config(&set);
> -	plane->fb = tmpfb;
>  
>  	kfree(connector_list);
>  	return ret;
> -- 
> 1.9.2
> 
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-- 
Matt Roper
Graphics Software Engineer
IoTG Platform Enabling & Development
Intel Corporation
(916) 356-2795