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

[Dan Carpenter]

On Fri, Aug 28, 2020 at 12:53:32PM +0300, Jani Nikula wrote:
> 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)?

A default switch is going to be unreachable as well...  But adding a
return 0 is the older more traditional ways way of marking these so
people and checkers are used to it.

regards,
dan carpenter