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

[Russell King - ARM Linux]

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.

There's a bigger issue here - if it's possible for drm_crtc_helper_set_config()
to be called with set->fb set but set->mode NULL, then we overwrite
set->fb to NULL.  Again, that results in a lost reference.

For the time being, I'm using this patch, which solves my dropped
refcount problem, and marks the other possible dropped reference.
Either that check needs to be removed or it needs to properly drop
the refcount on the fb before 'losing' the reference to it.

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index dd64a06..774d7a6 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -2014,13 +2014,16 @@ int drm_mode_set_config_internal(struct drm_mode_set *set)
 
 	old_fb = crtc->fb;
 	fb = set->fb;
+	if (fb)
+		drm_framebuffer_reference(fb);
 
 	ret = crtc->funcs->set_config(set);
 	if (ret == 0) {
 		if (old_fb)
 			drm_framebuffer_unreference(old_fb);
+	} else {
 		if (fb)
-			drm_framebuffer_reference(fb);
+			drm_framebuffer_unreference(fb);
 	}
 
 	return ret;
diff --git a/drivers/gpu/drm/drm_crtc_helper.c b/drivers/gpu/drm/drm_crtc_helper.c
index 7b2d378..0d18fb2 100644
--- a/drivers/gpu/drm/drm_crtc_helper.c
+++ b/drivers/gpu/drm/drm_crtc_helper.c
@@ -299,6 +299,8 @@ void drm_helper_disable_unused_functions(struct drm_device *dev)
 				(*crtc_funcs->disable)(crtc);
 			else
 				(*crtc_funcs->dpms)(crtc, DRM_MODE_DPMS_OFF);
+			if (crtc->fb)
+				drm_framebuffer_unreference(crtc->fb);
 			crtc->fb = NULL;
 		}
 	}
@@ -573,6 +575,7 @@ int drm_crtc_helper_set_config(struct drm_mode_set *set)
 
 	crtc_funcs = set->crtc->helper_private;
 
+	/* FIXME: this loses a refcount on the fb */
 	if (!set->mode)
 		set->fb = NULL;