linux-imx

drm/tegra: Fix crash caused by reference count imbalance

Commit d2307dea14a4 ("drm/atomic: use connector references (v3)") added
reference counting for DRM connectors and this caused a crash when
exercising system suspend on Tegra114 Dalmore.

The Tegra DSI driver implements a Tegra specific function,
tegra_dsi_connector_duplicate_state(), to duplicate the connector state
and destroys the state using the generic helper function,
drm_atomic_helper_connector_destroy_state(). Following commit
d2307dea14a4 ("drm/atomic: use connector references (v3)") there is
now an imbalance in the connector reference count because the Tegra
function to duplicate state does not take a reference when duplicating
the state information. However, the generic helper function to destroy
the state information assumes a reference has been taken and during
system suspend, when the connector state is destroyed, this leads to a
crash because we attempt to put the reference for an object that has
already been freed.

Fix this by calling __drm_atomic_helper_connector_duplicate_state() from
tegra_dsi_connector_duplicate_state() to ensure that we take a reference
on a connector if crtc is set. Note that this will also copy the
connector state a 2nd time, but this should be harmless.

By fixing tegra_dsi_connector_duplicate_state() to take a reference,
although a crash was no longer seen, it was then observed that after
each system suspend-resume cycle, the reference would be one greater
than before the suspend-resume cycle. Following commit d2307dea14a4
("drm/atomic: use connector references (v3)"), it was found that we
also need to put the reference when calling the function
tegra_dsi_connector_reset() before freeing the state. Fix this by
updating tegra_dsi_connector_reset() to call the function
__drm_atomic_helper_connector_destroy_state() in order to put the
reference for the connector.

Fixes: d2307dea14a4 ("drm/atomic: use connector references (v3)")

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1463585856-16606-1-git-send-email-jonathanh@nvidia.com