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

[Dan Carpenter]

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...

   496          default:
   497                  return 0;
   498          }
   499  }

regards,
dan carpenter