On Wed, Apr 23, 2014 at 10:30:04AM +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.
>
> v4: Also fix up the handling of ->disable_plane in
> drm_plane_force_disable. The issue was that we didn't save plane->fb
> over the ->disable_plane call. Just paranoia, nothing relies on this.
>
> Cc: Thierry Reding <treding@xxxxxxxxxx>
> Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> Cc: Matt Roper <matthew.d.roper@xxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
> drivers/gpu/drm/drm_crtc.c | 13 +++++++------
> drivers/gpu/drm/drm_plane_helper.c | 16 ----------------
> 2 files changed, 7 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index d8b7099abece..f6633cb927bc 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -1145,16 +1145,17 @@ EXPORT_SYMBOL(drm_plane_cleanup);
> */
> void drm_plane_force_disable(struct drm_plane *plane)
> {
> + struct drm_framebuffer *old_fb = plane->fb;
> int ret;
>
> - if (!plane->fb)
> + if (!old_fb)
> return;
>
> ret = plane->funcs->disable_plane(plane);
> if (ret)
> DRM_ERROR("failed to disable plane with busy fb\n");
> /* disconnect the plane from the fb and crtc: */
> - __drm_framebuffer_unreference(plane->fb);
> + __drm_framebuffer_unreference(old_fb);
> plane->fb = NULL;
> plane->crtc = NULL;
> }
So if disable_plane() fails here, we'd still be scanning out of old_fb,
yet we deref it...that doesn't seem quite right.
As you mention, the changes here are just for paranoia since we can't
hit failure here as far as I know; force_disable() is only called in
drm_framebuffer_remove() (where a CRTC loop has already taken care of
removing the framebuffer from the primary plane) and
drm_fb_helper_restore_fbdev_mode() (where we explicitly skip primary
planes). Since only primary planes can fail to be disabled, I'd be
inclined to just add a BUG_ON(plane->type == DRM_PLANE_TYPE_PRIMARY) at
the start of this function with a comment update explaining that it
can't be called on primary planes, and a BUG_ON(ret) since
non-primary planes should never fail to be disabled.
...
> @@ -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 the second half of this comment (explaining why we call
set_config() directly rather than calling
drm_mode_set_config_internal()) still has some value.
Matt
> - tmpfb = plane->fb;
> ret = crtc->funcs->set_config(&set);
> - plane->fb = tmpfb;
>
> kfree(connector_list);
> return ret;
> --
> 1.9.2
>
--
Matt Roper
Graphics Software Engineer
IoTG Platform Enabling & Development
Intel Corporation
(916) 356-2795
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