[Intel-gfx] [PATCH] drm/fb-helper: Use proper plane mask for fb cleanup

Matt Roper matthew.d.roper at intel.com
Fri Dec 18 17:27:01 PST 2015

pan_display_atomic() calls drm_atomic_clean_old_fb() to sanitize the
legacy FB fields (plane->fb and plane->old_fb).  However it was building
the plane mask to pass to this function incorrectly (the bitwise OR was
using plane indices rather than plane masks).  The end result was that
sometimes the legacy pointers would become out of sync with the atomic
pointers.  If another operation tried to re-set the same FB onto the
plane, we might end up with the pointers back in sync, but improper
reference counts, which would eventually lead to system crashes when we
accessed a pointer to a prematurely-destroyed FB.

The cause here was a very subtle bug introduced in commit:

        commit 07d3bad6c1210bd21e85d084807ef4ee4ac43a78
        Author: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
        Date:   Wed Nov 11 11:29:11 2015 +0100

            drm/core: Fix old_fb handling in pan_display_atomic.

I found the crashes were most easily reproduced (on i915 at least) by
starting X and then VT switching to a VT that wasn't running a console
instance...the sequence of vt/fbcon entries that happen in that case
trigger a reference count mismatch and crash the system.

Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93313
Signed-off-by: Matt Roper <matthew.d.roper at intel.com>
 drivers/gpu/drm/drm_fb_helper.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 69cbab5..1e103c4 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1251,7 +1251,7 @@ retry:
 			goto fail;
 		plane = mode_set->crtc->primary;
-		plane_mask |= drm_plane_index(plane);
+		plane_mask |= (1 << drm_plane_index(plane));
 		plane->old_fb = plane->fb;

More information about the Intel-gfx mailing list