linux-kernel - RE: [PATCH] USB: serial: io_edgeport: mark expected switch fall-throughs

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Date:   Mon, 30 Oct 2017 11:54:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Bjørn Mork' <bjorn@...k.no>,
        "Gustavo A. R. Silva" <garsilva@...eddedor.com>
CC:     Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] USB: serial: io_edgeport: mark expected switch
 fall-throughs

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.

	David