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

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Date:   Sat, 28 Oct 2017 12:56:51 +0200
From:   Bjørn Mork <bjorn@...k.no>
To:     "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-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: serial: io_edgeport: mark expected switch fall-throughs

"Gustavo A. R. Silva" <garsilva@...eddedor.com> writes:

> 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.

Bjørn