[PATCH RFC 2/8] DRM: Armada: Add Armada DRM driver

[Russell King - ARM Linux]

On Fri, Jun 14, 2013 at 03:53:41PM +0200, Daniel Vetter wrote:
> On Thu, Jun 13, 2013 at 12:50:16PM +0100, Russell King - ARM Linux wrote:
> > On Thu, Jun 13, 2013 at 12:19:03PM +0100, Russell King - ARM Linux wrote:
> > > The deeper I look, the more bugs there seem to be in this DRM stuff,
> > > and I'm continuing to look because I'm chasing a framebuffer refcount
> > > bug.
> > 
> > So, this refcount bug - I think I've just found it.  This is the flow of
> > references to the new fb on mode set:
> > 
> > drm_mode_setcrtc():
> >                         fb = drm_framebuffer_lookup(dev, crtc_req->fb_id);
> >         set.fb = fb;
> >         ret = drm_mode_set_config_internal(&set);
> > drm_mode_set_config_internal():
> >         fb = set->fb;
> >         ret = crtc->funcs->set_config(set);
> > drm_crtc_helper_set_config():
> >                         old_fb = set->crtc->fb;
> >                         set->crtc->fb = set->fb;
> >                         if (!drm_crtc_helper_set_mode(set->crtc, set->mode,
> >                                                       set->x, set->y,
> >                                                       old_fb)) {
> >                 drm_helper_disable_unused_functions(dev);
> > drm_helper_disable_unused_functions():
> >         list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
> >                 crtc->enabled = drm_helper_crtc_in_use(crtc);
> >                 if (!crtc->enabled) {
> >                         crtc->fb = NULL;
> > 		}
> > 	}
> > back to drm_mode_set_config_internal():
> >         if (ret == 0) {
> >                 if (fb)
> >                         drm_framebuffer_reference(fb);
> > back to drm_mode_setcrtc():
> >         if (fb)
> >                 drm_framebuffer_unreference(fb);
> > 
> > Assuming success all the way through, what happens when a CRTC is unused
> > is:
> > 
> > 1. We obtain a reference in drm_mode_setcrtc() via the lookup.
> > 2. We set the mode
> > 3. In trying to set the mode, we discover that all connectors for the CRTC
> >    are in the disconnected state, and so we disable the CRTC
> > 4. We set crtc->fb to NULL
> > 5. back in drm_mode_set_config_internal(), we take a reference on the
> >    framebuffer irrespective of this.
> > 6. back in drm_mode_setcrtc(), we drop the original reference caused by
> >    the lookup.
> > 
> > We now have a framebuffer with a reference count incremented by one but
> > no actual reference to it - the CRTC's reference is completely lost by
> > the action of drm_helper_disable_unused_functions().
> > 
> > You could argue that it's something the driver should deal with - fine,
> > but what if it only implements the DPMS method?  Should it drop a
> > reference to the framebuffer when DPMS instructs it to turn off?  Surely
> > not, because that means when DPMS turns stuff back on you're missing a
> > refcount.
> > 
> > Are drivers required to implement a disable function and cater for the
> > imbalance in the upper layers of code?  If so, this is not a clean
> > design.
> 
> Yep, if your driver grabs additional references (underlying gem object,
> pinning, whatever) you need to wire up your own ->disable hook to drop
> those. Note that for truly dumb kms drivers which only ever allocate an
> fb, the upper layer actually _does_ take care of all the refcounting.

Look again at what I said above - that is _not_ the case.  My above
analysis is totally without considering any refcounting that the
driver itself may or may not do.  If the driver does no refcounting on
the framebuffer object, then the leak is present.