From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matt Roper Subject: Re: [PATCH 1/2] drm: Simplify fb refcounting rules around ->update_plane Date: Wed, 23 Apr 2014 07:45:19 -0700 Message-ID: <20140423144519.GU1063@intel.com> References: <1398241805-24564-1-git-send-email-daniel.vetter@ffwll.ch> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Return-path: Received: from mga03.intel.com (mga03.intel.com [143.182.124.21]) by gabe.freedesktop.org (Postfix) with ESMTP id C7D0D6EAC9 for ; Wed, 23 Apr 2014 07:42:36 -0700 (PDT) Content-Disposition: inline In-Reply-To: <1398241805-24564-1-git-send-email-daniel.vetter@ffwll.ch> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" To: Daniel Vetter Cc: Thierry Reding , DRI Development List-Id: dri-devel@lists.freedesktop.org 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 > Cc: Ville Syrj=E4l=E4 > Cc: Matt Roper > Signed-off-by: Daniel Vetter > --- > 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 =3D plane->fb; > int ret; > = > - if (!plane->fb) > + if (!old_fb) > return; > = > ret =3D 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 =3D NULL; > plane->crtc =3D 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 =3D=3D 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 *plan= e, struct drm_crtc *crtc, > set.connectors =3D connector_list; > set.num_connectors =3D 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 =3D plane->fb; > ret =3D crtc->funcs->set_config(&set); > - plane->fb =3D tmpfb; > = > kfree(connector_list); > return ret; > -- = > 1.9.2 > = -- = Matt Roper Graphics Software Engineer IoTG Platform Enabling & Development Intel Corporation (916) 356-2795