Re: [PATCH v2] USB: serial: io_edgeport: mark expected switch fall-throughs

From: Johan Hovold
Date: Thu May 02 2019 - 11:15:15 EST

On Thu, May 02, 2019 at 04:47:29PM +0200, Greg Kroah-Hartman wrote:
> On Thu, May 02, 2019 at 04:40:41PM +0200, Greg Kroah-Hartman wrote:
> > On Thu, May 02, 2019 at 09:28:37AM -0500, Gustavo A. R. Silva wrote:
> > >
> > >
> > > On 5/2/19 8:56 AM, Johan Hovold wrote:
> > > > On Thu, May 02, 2019 at 08:22:30AM -0500, Gustavo A. R. Silva wrote:
> > > >>
> > > >>
> > > >> On 5/2/19 5:26 AM, Johan Hovold wrote:
> > > >>> On Wed, May 01, 2019 at 04:33:29PM -0500, Gustavo A. R. Silva wrote:
> > > >>>> In preparation to enabling -Wimplicit-fallthrough, mark switch
> > > >>>> cases where we are expecting to fall through.
> > > >>>>
> > > >>>> This patch fixes the following warnings:
> > > >>>>
> > > >>>> drivers/usb/serial/io_edgeport.c: In function âprocess_rcvd_dataâ:
> > > >>>> drivers/usb/serial/io_edgeport.c:1750:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
> > > >>>> if (bufferLength == 0) {
> > > >>>> ^
> > > >>>> drivers/usb/serial/io_edgeport.c:1755:3: note: here
> > > >>>> case EXPECT_HDR2:
> > > >>>> ^~~~
> > > >>>> drivers/usb/serial/io_edgeport.c:1810:8: warning: this statement may fall through [-Wimplicit-fallthrough=]
> > > >>>> if (bufferLength == 0) {
> > > >>>> ^
> > > >>>> drivers/usb/serial/io_edgeport.c:1816:3: note: here
> > > >>>> case EXPECT_DATA: /* Expect data */
> > > >>>> ^~~~
> > > >>>>
> > > >>>> Warning level 3 was used: -Wimplicit-fallthrough=3
> > > >>>>
> > > >>>> Notice that, in this particular case, the code comments are modified
> > > >>>> in accordance with what GCC is expecting to find.
> > > >>>>
> > > >>>> This patch is part of the ongoing efforts to enable
> > > >>>> -Wimplicit-fallthrough.
> > > >>>>
> > > >>>> Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> > > >>>> ---
> > > >>>> Changes in v2:
> > > >>>> - Warning level 3 is now used: -Wimplicit-fallthrough=3
> > > >>>> instead of warning level 2.
> > > >>>> - All warnings in the switch statement are addressed now.
> > > >>>>
> > > >>>> Notice that these are the last remaining fall-through warnings
> > > >>>> in the USB subsystem. :)
> > > >>>
> > > >>>> drivers/usb/serial/io_edgeport.c | 3 ++-
> > > >>>> 1 file changed, 2 insertions(+), 1 deletion(-)
> > > >>>>
> > > >>>> diff --git a/drivers/usb/serial/io_edgeport.c b/drivers/usb/serial/io_edgeport.c
> > > >>>> index 4ca31c0e4174..7ad10328f4e2 100644
> > > >>>> --- a/drivers/usb/serial/io_edgeport.c
> > > >>>> +++ b/drivers/usb/serial/io_edgeport.c
> > > >>>> @@ -1751,7 +1751,7 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
> > > >>>> edge_serial->rxState = EXPECT_HDR2;
> > > >>>> break;
> > > >>>> }
> > > >>>> - /* otherwise, drop on through */
> > > >>>> + /* Fall through - otherwise, drop on through */
> > > >>>> case EXPECT_HDR2:
> > > >>>> edge_serial->rxHeader2 = *buffer;
> > > >>>> ++buffer;
> > > >>>> @@ -1813,6 +1813,7 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
> > > >>>> }
> > > >>>> /* Else, drop through */
> > > >>>> }
> > > >>>> + /* Fall through */
> > > >>>> case EXPECT_DATA: /* Expect data */
> > > >>>
> > > >>> Looks like you forgot to take the original review feedback you got into
> > > >>> account:
> > > >>>
> > > >>> https://lkml.kernel.org/r/87k1zf4k24.fsf@xxxxxxxxxxxxxxxxx
> > > >>>
> > > >>
> > > >> Oh, the thing is that the fall-through comments have to be placed at
> > > >> the very bottom of the case. Also, based on that feedback, this time
> > > >> I left the "Else, drop through" comment in place, so people can be
> > > >> informed that such fall-through is conditional.
> > > >>
> > > >> What do you think about this:
> > > >>
> > > >> diff --git a/drivers/usb/serial/io_edgeport.c b/drivers/usb/serial/io_edgeport.c
> > > >> index 4ca31c0e4174..52f27fc82563 100644
> > > >> --- a/drivers/usb/serial/io_edgeport.c
> > > >> +++ b/drivers/usb/serial/io_edgeport.c
> > > >> @@ -1751,7 +1751,7 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
> > > >> edge_serial->rxState = EXPECT_HDR2;
> > > >> break;
> > > >> }
> > > >> - /* otherwise, drop on through */
> > > >> + /* Fall through - otherwise, drop on through */
> > > >> case EXPECT_HDR2:
> > > >> edge_serial->rxHeader2 = *buffer;
> > > >> ++buffer;
> > > >> @@ -1813,6 +1813,11 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
> > > >> }
> > > >> /* Else, drop through */
> > > >> }
> > > >> + /* Beware that, currently, there are at least three
> > > >> + * break statements in this case block, so the
> > > >> + * fall-through marked below is NOT unconditional.
> > > >> + */
> > > >> + /* Fall through */
> > > >> case EXPECT_DATA: /* Expect data */
> > > >> if (bufferLength < edge_serial->rxBytesRemaining) {
> > > >> rxLen = bufferLength;
> > > >
> > > > It's better than v2, but I thought you said you were gonna look into
> > > > restructuring the code to maintain (or even improve) readability?
> > > >
> > >
> > > At first, I thought about that, but now I don't think that's realistic.
> > > I'd turn the if-else into a switch, and based on the history of feedback
> > > on this patch, we will end up having the same complains about the break
> > > statements in that new switch and the possibility of a fall-through to
> > > case EXPECT_DATA. At the end I would still have to add a comment explaining
> > > that the last fall-through mark in unconditional.
> >
> > I love it how no one is blaming the original author of this code (i.e.
> > me...)
> >
> > Let me see if I can fix it up to be more "sane", this is my fault.
>
> How about the following patch? Johan, this look nicer to you? It makes
> more sense to me.

Hard to say... :) Less indentation is good, but the current flow seems
more in line with the comment preceding the if-statement

/* Process depending on whether this header is
* data or status */

if (IS_CMD_STAT_HDR(edge_serial->rxHeader1)) {
a;
break;
} else {
b;
}
case EXPECT_DATA: /* Expect data */

which sort of gets lost if you just replace the else clause with b.

But it still an improvement, let's go with it.

> And in looking at the history, I can't claim total credit for this
> monstrosity, it was originally written by someone else, I just "cleaned
> it up" back in 2001, to get it into mergable shape. Clearly "mergable
> shape" was much looser back then :)

Heh.

> diff --git a/drivers/usb/serial/io_edgeport.c b/drivers/usb/serial/io_edgeport.c
> index 4ca31c0e4174..732081b3718f 100644
> --- a/drivers/usb/serial/io_edgeport.c
> +++ b/drivers/usb/serial/io_edgeport.c
> @@ -1751,7 +1751,8 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
> edge_serial->rxState = EXPECT_HDR2;
> break;
> }
> - /* otherwise, drop on through */
> + /* Fall through */
> +

I think the comment should go immediately before the case (no newline)
here and below though.

> case EXPECT_HDR2:

Thanks,
Johan