Re: [gustavoars-linux:testing/fallthrough 144/560] drivers/gpu/drm/drm_dp_helper.c:492 drm_dp_downstream_max_bpc() warn: ignoring unreachable code.

[Gustavo A. R. Silva <gustavoars@kernel.org>]

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Hi,

I think there is room for improvement here. Please, see my comments
below...

On Tue, Jul 07, 2020 at 05:07:17PM +0300, Dan Carpenter wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/gustavoars/linux.git testing/fallthrough
> head:   d7f10c2411ed9283df8d763139de9468b88c89ce
> commit: 62708a516a6b158e213c46ff731260cb988dbc11 [144/560] drm: Use fallthrough pseudo-keyword
> config: i386-randconfig-m021-20200703 (attached as .config)
> compiler: gcc-9 (Debian 9.3.0-14) 9.3.0
> 
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> 
> smatch warnings:
> drivers/gpu/drm/drm_dp_helper.c:492 drm_dp_downstream_max_bpc() warn: ignoring unreachable code.
> 
> # https://git.kernel.org/pub/scm/linux/kernel/git/gustavoars/linux.git/commit/?id=62708a516a6b158e213c46ff731260cb988dbc11
> git remote add gustavoars-linux https://git.kernel.org/pub/scm/linux/kernel/git/gustavoars/linux.git
> git remote update gustavoars-linux
> git checkout 62708a516a6b158e213c46ff731260cb988dbc11
> vim +492 drivers/gpu/drm/drm_dp_helper.c
> 
> 7529d6af1cd16b Mika Kahola         2016-09-09  464  int drm_dp_downstream_max_bpc(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
> 7529d6af1cd16b Mika Kahola         2016-09-09  465  			      const u8 port_cap[4])
> 7529d6af1cd16b Mika Kahola         2016-09-09  466  {
> 7529d6af1cd16b Mika Kahola         2016-09-09  467  	int type = port_cap[0] & DP_DS_PORT_TYPE_MASK;
> 7529d6af1cd16b Mika Kahola         2016-09-09  468  	bool detailed_cap_info = dpcd[DP_DOWNSTREAMPORT_PRESENT] &
> 7529d6af1cd16b Mika Kahola         2016-09-09  469  		DP_DETAILED_CAP_INFO_AVAILABLE;
> 7529d6af1cd16b Mika Kahola         2016-09-09  470  	int bpc;
> 7529d6af1cd16b Mika Kahola         2016-09-09  471  
> 7529d6af1cd16b Mika Kahola         2016-09-09  472  	if (!detailed_cap_info)
> 7529d6af1cd16b Mika Kahola         2016-09-09  473  		return 0;
> 7529d6af1cd16b Mika Kahola         2016-09-09  474  
> 7529d6af1cd16b Mika Kahola         2016-09-09  475  	switch (type) {
> 7529d6af1cd16b Mika Kahola         2016-09-09  476  	case DP_DS_PORT_TYPE_VGA:
> 7529d6af1cd16b Mika Kahola         2016-09-09  477  	case DP_DS_PORT_TYPE_DVI:
> 7529d6af1cd16b Mika Kahola         2016-09-09  478  	case DP_DS_PORT_TYPE_HDMI:
> 7529d6af1cd16b Mika Kahola         2016-09-09  479  	case DP_DS_PORT_TYPE_DP_DUALMODE:
> 7529d6af1cd16b Mika Kahola         2016-09-09  480  		bpc = port_cap[2] & DP_DS_MAX_BPC_MASK;
> 7529d6af1cd16b Mika Kahola         2016-09-09  481  
> 7529d6af1cd16b Mika Kahola         2016-09-09  482  		switch (bpc) {
> 7529d6af1cd16b Mika Kahola         2016-09-09  483  		case DP_DS_8BPC:
> 7529d6af1cd16b Mika Kahola         2016-09-09  484  			return 8;
> 7529d6af1cd16b Mika Kahola         2016-09-09  485  		case DP_DS_10BPC:
> 7529d6af1cd16b Mika Kahola         2016-09-09  486  			return 10;
> 7529d6af1cd16b Mika Kahola         2016-09-09  487  		case DP_DS_12BPC:
> 7529d6af1cd16b Mika Kahola         2016-09-09  488  			return 12;
> 7529d6af1cd16b Mika Kahola         2016-09-09  489  		case DP_DS_16BPC:
> 7529d6af1cd16b Mika Kahola         2016-09-09  490  			return 16;
> 7529d6af1cd16b Mika Kahola         2016-09-09  491  		}
> 62708a516a6b15 Gustavo A. R. Silva 2020-07-02 @492  		fallthrough;
>                                                                 ^^^^^^^^^^^

Notice that the statement above and the code below is actually reachable
if _bpc_ doesn't match any of the switch cases.

Thanks
--
Gustavo

> 
> 7529d6af1cd16b Mika Kahola         2016-09-09  493  	default:
> 7529d6af1cd16b Mika Kahola         2016-09-09  494  		return 0;
> 7529d6af1cd16b Mika Kahola         2016-09-09  495  	}
> 7529d6af1cd16b Mika Kahola         2016-09-09  496  }
> 
> ---
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