[PATCH] drm/fbdev: Update legacy plane->fb refcounting for atomic restore

[Daniel Vetter]

From: Matt Roper <matthew.d.roper at intel.com>

Starting with commit

        commit 28cc504e8d52248962f5b485bdc65f539e3fe21d
        Author: Rob Clark <robdclark at gmail.com>
        Date:   Tue Aug 25 15:36:00 2015 -0400

            drm/i915: enable atomic fb-helper

I've been seeing some panics on i915 when the DRM master shuts down that appear
to be caused by using an already-freed framebuffer (i.e., we're unexpectedly
dropping our initial FB's reference count to 0 and freeing it, which causes a
crash when we try to restore it later).  Digging deeper, the state FB
refcounting is working as expected, but we seem to be missing proper
refcounting on the legacy plane->fb pointers in the new atomic fbdev code.

Tracking plane->old_fb and then doing a ref/unref at the end of the
fbdev restore like we do in the legacy ioctl's ensures we don't miscount
references on plane->fb and avoids the panics.

v2 from Daniel:

Really do what the atomic ioctl does:
- Also update plane->fb and plane->crtc.
- Clear out plane->old_fb on failures too.

Cc: Rob Clark <robdclark at gmail.com>
Cc: intel-gfx at lists.freedesktop.org
Signed-off-by: Matt Roper <matthew.d.roper at intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/drm_fb_helper.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 64fc5ca8fda2..1b564a69f561 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -352,6 +352,8 @@ retry:
 	drm_for_each_plane(plane, dev) {
 		struct drm_plane_state *plane_state;
 
+		plane->old_fb = plane->fb;
+
 		plane_state = drm_atomic_get_plane_state(state, plane);
 		if (IS_ERR(plane_state)) {
 			ret = PTR_ERR(plane_state);
@@ -382,6 +384,20 @@ retry:
 	}
 
 	ret = drm_atomic_commit(state);
+
+	drm_for_each_plane(plane, dev) {
+		if (ret == 0) {
+			struct drm_framebuffer *new_fb = plane->state->fb;
+			if (new_fb)
+				drm_framebuffer_reference(new_fb);
+			plane->fb = new_fb;
+			plane->crtc = plane->state->crtc;
+
+			if (plane->old_fb)
+				drm_framebuffer_unreference(plane->old_fb);
+		}
+	}
+
 	if (ret != 0)
 		goto fail;
 
-- 
2.5.1