[PATCHv5 4/6] drm/omap: fix incorrect union usage

[Tony Lindgren]

* Sebastian Reichel <sebastian.reichel at collabora.com> [181121 16:09]:
> The DSI encoder sets dssdev->ops->dsi.set_config, which is stored at the
> same offset as dssdev->ops->hdmi.set_hdmi_mode. The code in omap_encoder
> only checks if dssdev->ops->hdmi.set_hdmi_mode is NULL. Due to the way
> union works, it won't be NULL if dsi.set_config is set. This means
> dsi_set_config will be called with config=hdmi_mode=false=NULL parameter
> resulting in a NULL dereference. Also the dereference happens while
> console is locked, so kernel hangs without any debug output without
> "fb.lockless_register_fb=1" parameter.
> 
> This restructures the code, so that the HDMI mode is only configured
> for HDMI output types. The new function also has a safe-guard directly
> before accessing the union, that can be optimized away by the compiler
> when the function is inlined and HDMI type has already been checked.
> 
> Fixes: 83910ad3f51fb ("drm/omap: Move most omap_dss_driver operations to omap_dss_device_ops")
> Signed-off-by: Sebastian Reichel <sebastian.reichel at collabora.com>

Works for me:

Tested-by: Tony Lindgren <tony at atomide.com>