drm: Add consistency check for page-flipping

Driver implementations of the drm_crtc's .page_flip() function are
required to update the crtc->fb field on success to reflect that the new
framebuffer is now in use. This is important to keep reference counting
on the framebuffers balanced.

While at it, document this requirement to keep others from falling into
the same trap.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

2 files changed, 13 insertions, 0 deletions

diff --git a/Documentation/DocBook/drm.tmpl b/Documentation/DocBook/drm.tmpl
index 51e1904ac4c7..a6428ddfcfc2 100644
--- a/Documentation/DocBook/drm.tmpl
+++ b/Documentation/DocBook/drm.tmpl
@@ -1161,6 +1161,12 @@ int max_width, max_height;</synopsis>
             any new rendering to the frame buffer until the page flip completes.
           </para>
           <para>
+            If a page flip can be successfully scheduled the driver must set the
+            <code>drm_crtc-&lt;fb</code> field to the new framebuffer pointed to
+            by <code>fb</code>. This is important so that the reference counting
+            on framebuffers stays balanced.
+          </para>
+          <para>
             If a page flip is already pending, the
             <methodname>page_flip</methodname> operation must return
             -<errorname>EBUSY</errorname>.
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 781aef524b3b..3bdf2a650d9c 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -3792,6 +3792,13 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
                 /* Keep the old fb, don't unref it. */
                 old_fb = NULL;
         } else {
+                /*
+                 * Warn if the driver hasn't properly updated the crtc->fb
+                 * field to reflect that the new framebuffer is now used.
+                 * Failing to do so will screw with the reference counting
+                 * on framebuffers.
+                 */
+                WARN_ON(crtc->fb != fb);
                 /* Unref only the old framebuffer. */
                 fb = NULL;
         }