[PATCH] drm/display/dsc: Refactor MST DSC Determination Policy

[Dan Carpenter]

Hi Fangzhi,

kernel test robot noticed the following build warnings:

https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Fangzhi-Zuo/drm-display-dsc-Refactor-MST-DSC-Determination-Policy/20240920-014114
base:   git://anongit.freedesktop.org/drm-intel for-linux-next
patch link:    https://lore.kernel.org/r/20240919173944.256887-1-Jerry.Zuo%40amd.com
patch subject: [PATCH] drm/display/dsc: Refactor MST DSC Determination Policy
config: microblaze-randconfig-r071-20240922 (https://download.01.org/0day-ci/archive/20240923/202409231002.bMP89Ipm-lkp@intel.com/config)
compiler: microblaze-linux-gcc (GCC) 14.1.0

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp at intel.com>
| Reported-by: Dan Carpenter <dan.carpenter at linaro.org>
| Closes: https://lore.kernel.org/r/202409231002.bMP89Ipm-lkp@intel.com/

smatch warnings:
drivers/gpu/drm/display/drm_dp_mst_topology.c:6046 drm_dp_mst_dsc_aux_for_port() warn: variable dereferenced before check 'port' (see line 6035)

vim +/port +6046 drivers/gpu/drm/display/drm_dp_mst_topology.c

cc707186414576 drivers/gpu/drm/display/drm_dp_mst_topology.c Fangzhi Zuo      2024-09-19  6033  void drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port)
c2bc1b6eabe65d drivers/gpu/drm/drm_dp_mst_topology.c         David Francis    2019-08-26  6034  {
cc707186414576 drivers/gpu/drm/display/drm_dp_mst_topology.c Fangzhi Zuo      2024-09-19 @6035  	struct drm_dp_mst_topology_mgr *mgr = port->mgr;
                                                                                                                                              ^^^^^^^^^
The patch adds an unchecked dereference

cc707186414576 drivers/gpu/drm/display/drm_dp_mst_topology.c Fangzhi Zuo      2024-09-19  6036  	struct drm_dp_mst_port *immediate_upstream_port = NULL;
cc707186414576 drivers/gpu/drm/display/drm_dp_mst_topology.c Fangzhi Zuo      2024-09-19  6037  	struct drm_dp_mst_port *fec_port = NULL;
cc707186414576 drivers/gpu/drm/display/drm_dp_mst_topology.c Fangzhi Zuo      2024-09-19  6038  	struct drm_dp_mst_port *dsc_port = NULL;
cc707186414576 drivers/gpu/drm/display/drm_dp_mst_topology.c Fangzhi Zuo      2024-09-19  6039  	struct drm_dp_aux *upstream_aux;
53965dbe5396d2 drivers/gpu/drm/drm_dp_mst_topology.c         Paul E. McKenney 2020-02-19  6040  	struct drm_dp_desc desc = {};
cc707186414576 drivers/gpu/drm/display/drm_dp_mst_topology.c Fangzhi Zuo      2024-09-19  6041  	bool end_has_dpcd = (port->dpcd_rev > 0);
cc707186414576 drivers/gpu/drm/display/drm_dp_mst_topology.c Fangzhi Zuo      2024-09-19  6042  	u8 endpoint_dsc = 0;
cc707186414576 drivers/gpu/drm/display/drm_dp_mst_topology.c Fangzhi Zuo      2024-09-19  6043  	u8 upstream_dsc;
cc707186414576 drivers/gpu/drm/display/drm_dp_mst_topology.c Fangzhi Zuo      2024-09-19  6044  	u8 fec_cap;
c2bc1b6eabe65d drivers/gpu/drm/drm_dp_mst_topology.c         David Francis    2019-08-26  6045  
c2bc1b6eabe65d drivers/gpu/drm/drm_dp_mst_topology.c         David Francis    2019-08-26 @6046  	if (!port)
                                                                                                            ^^^^^
But the old code assumed port could be NULL.

-- 
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