This is a tricky story of the new atomic state handling and the legacy
code fighting over each another. The bug at hand is an underrun of the
framebuffer reference with subsequent hilarity caused by the load
detect code. Which is peculiar since the the exact same code works
fine as the implementation of the legacy setcrtc ioctl.
Let's look at the ingredients:
- Currently our code is a crazy mix of legacy modeset interfaces to
set the parameters and half-baked atomic state tracking underneath.
While this transition is going we're using the transitional plane
helpers to update the atomic side (drm_plane_helper_disable/update
and friends), i.e. plane->state->fb. Since the state structure owns
the fb those functions take care of that themselves.
The legacy state (specifically crtc->primary->fb) is still managed
by the old code (and mostly by the drm core), with the fb reference
counting done by callers (core drm for the ioctl or the i915 load
detect code). The relevant commit is
commit ea2c67bb4affa84080c616920f3899f123786e56
Author: Matt Roper <matthew.d.roper@intel.com>
Date: Tue Dec 23 10:41:52 2014 -0800
drm/i915: Move to atomic plane helpers (v9)
- drm_plane_helper_disable has special code to handle multiple calls
in a row - it checks plane->crtc == NULL and bails out. This is to
match the proper atomic implementation which needs the crtc to get
at the implied locking context atomic updates always need. See
commit acf24a395c5a9290189b080383564437101d411c
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date: Tue Jul 29 15:33:05 2014 +0200
drm/plane-helper: transitional atomic plane helpers
- The universal plane code split out the implicit primary plane from
the CRTC into it's own full-blown drm_plane object. As part of that
the setcrtc ioctl (which updated both the crtc mode and primary
plane) learned to set crtc->primary->crtc on modeset to make sure
the plane->crtc assignments statate up to date in
commit e13161af80c185ecd8dc4641d0f5df58f9e3e0af
Author: Matt Roper <matthew.d.roper@intel.com>
Date: Tue Apr 1 15:22:38 2014 -0700
drm: Add drm_crtc_init_with_planes() (v2)
Unfortunately we've forgotten to update the load detect code. Which
wasn't a problem since the load detect modeset is temporary and
always undone before we drop the locks.
- Finally there is a organically grown history (i.e. don't ask) around
who sets the legacy plane->fb for the various driver entry points.
Originally updating that was the drivers duty, but for almost all
places we've moved that (plus updating the refcounts) into the core.
Again the exception is the load detect code.
Taking all together the following happens:
- The load detect code doesn't set crtc->primary->crtc. This is only
really an issue on crtcs never before used or when userspace
explicitly disabled the primary plane.
- The plane helper glue code short-circuits because of that and leaves
a non-NULL fb behind in plane->state->fb and plane->fb. The state
fb isn't a real problem (it's properly refcounted on its own), it's
just the canary.
- Load detect code drops the reference for that fb, but doesn't set
plane->fb = NULL. This is ok since it's still living in that old
world where drivers had to clear the pointer but the core/callers
handled the refcounting.
- On the next modeset the drm core notices plane->fb and takes care of
refcounting it properly by doing another unref. This drops the
refcount to zero, leaving state->plane now pointing at freed memory.
- intel_plane_duplicate_state still assume it owns a reference to that
very state->fb and bad things start to happen.
Fix this all by applying the same duct-tape as for the legacy setcrtc
ioctl code and set crtc->primary->crtc properly.
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Paul Bolle <pebolle@tiscali.nl>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Paulo Zanoni <przanoni@gmail.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reported-and-tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>