[bug report] drm/dp: annotate implicit fall throughs

[Jani Nikula]

On Tue, 25 Aug 2020, Dan Carpenter <dan.carpenter at oracle.com> wrote:
> Hello Mathieu Malaterre,
>
> The patch e9c0c874711b: "drm/dp: annotate implicit fall throughs"
> from Jan 14, 2019, leads to the following static checker warning:
>
> 	drivers/gpu/drm/drm_dp_helper.c:495 drm_dp_downstream_max_bpc()
> 	warn: ignoring unreachable code.
>
> drivers/gpu/drm/drm_dp_helper.c
>    467  int drm_dp_downstream_max_bpc(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
>    468                                const u8 port_cap[4])
>    469  {
>    470          int type = port_cap[0] & DP_DS_PORT_TYPE_MASK;
>    471          bool detailed_cap_info = dpcd[DP_DOWNSTREAMPORT_PRESENT] &
>    472                  DP_DETAILED_CAP_INFO_AVAILABLE;
>    473          int bpc;
>    474  
>    475          if (!detailed_cap_info)
>    476                  return 0;
>    477  
>    478          switch (type) {
>    479          case DP_DS_PORT_TYPE_VGA:
>    480          case DP_DS_PORT_TYPE_DVI:
>    481          case DP_DS_PORT_TYPE_HDMI:
>    482          case DP_DS_PORT_TYPE_DP_DUALMODE:
>    483                  bpc = port_cap[2] & DP_DS_MAX_BPC_MASK;
>                                             ^^^^^^^^^^^^^^^^^^
> This is 0x3.
>
>    484  
>    485                  switch (bpc) {
>    486                  case DP_DS_8BPC:
>    487                          return 8;
>    488                  case DP_DS_10BPC:
>    489                          return 10;
>    490                  case DP_DS_12BPC:
>    491                          return 12;
>    492                  case DP_DS_16BPC:
>    493                          return 16;
>    494                  }
>    495                  fallthrough;
>
> This fallthrough is impossible.  Probably the way to work around the
> bogus warning is the change the fallthough to "return 0; /* impossible */"
> otherwise the fallthrough is sort of misleading...

Won't that be unreachable as well?

Maybe just add the default label to switch (bpc)?

BR,
Jani.


>
>    496          default:
>    497                  return 0;
>    498          }
>    499  }
>
> regards,
> dan carpenter
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-- 
Jani Nikula, Intel Open Source Graphics Center