Re: [PATCH] USB: serial: io_edgeport: mark expected switch fall-throughs

[Johan Hovold <johan@kernel.org>]

On Mon, Oct 30, 2017 at 11:54:33AM +0000, David Laight wrote:
> From: Bjørn Mork
> > Sent: 28 October 2017 11:57
> > > In preparation to enabling -Wimplicit-fallthrough, mark switch cases
> > > where we are expecting to fall through.
> > >
> > > Notice that in this particular case I replaced "...drop on through"
> > > comments with a proper "fall through" comment on its own line, which
> > > is what GCC is expecting to find.
> > 
> > Sounds to me like GCC is the wrong tool for this.  But I would of course
> > not mind if it was *just* the text.  However, as your patch cleary
> > shows, the simplified logic leads to real problems:
> > 
> > > @@ -1819,8 +1819,8 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
> > >  					edge_serial->rxState = EXPECT_DATA;
> > >  					break;
> > >  				}
> > > -				/* Else, drop through */
> > >  			}
> > > +			/* fall through */
> > >  		case EXPECT_DATA: /* Expect data */
> > >  			if (bufferLength < edge_serial->rxBytesRemaining) {
> > >  				rxLen = bufferLength;
> > 
> > 
> > The original comment clearly marked a *conditional* fall through at the
> > correct place.  Your patch makes it appear as if there is an
> > unconditional fall through here.  There is not.  The fallthrough only
> > applies to one of a number of nested if blocks. There are no less than
> > 3 break statements in the same case block.
> > 
> > Not a big deal maybe, just as the lack of any "fall through" comment
> > isn't a big deal in the first place.  But this change is clearly making
> > this code harder to read, and the change is therefore harmful IMHO.
> > 
> > If you can't make -Wimplicit-fallthrough work without removing such
> > precise fallthrough markings, then my proposal is to drop it and use
> > some other tool.
> 
> Just remove the 'else' after the 'break' further up.

Yeah, that might be a good way to resolve this. I was gonna suggest
adding the "fall though" comment before the case while keeping the
"Else, drop through" comment in the branch, but removing the else might
be better.

This code is pretty hard to read as is and could really use some clean
up...

Thanks,
Johan