Re: [dpdk-kmods] linux/igb_uio: fix build for switch fall through

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 15 Dec 2021 21:04:30 +0000
Ferruh Yigit <ferruh.yigit@intel.com> wrote:

> On 12/15/2021 7:20 PM, Stephen Hemminger wrote:
> > On Wed, 15 Dec 2021 18:48:59 +0000
> > Ferruh Yigit <ferruh.yigit@intel.com> wrote:
> >   
> >> Linux is using '-Wimplicit-fallthrough=5' compiler option, which doesn't
> >> take any fall through comments into account but only uses compiler
> >> 'fallthrough' attribute to document fall through action is intended.
> >>
> >> "falls through" comment was used in the code which is causing a build
> >> error now, this patch converts comment to the 'fallthrough' macro
> >> defined in the Linux.
> >>
> >> To cover the case where Linux version doesn't have the macro, defined it
> >> in the compatibility header too.
> >>
> >> Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>  
> > 
> > Are you sure that fixes it? because the comment in the macro is typically
> > not visible in a macro expansion.
> >    Since in most case Linux uses gcc why not use the gcc attribute
> > 
> >   __attribute__ ((fallthrough))  
> 
> Hi Stephen,
> 
> That is the intention already.
> 
> Patch is using the Linux kernel defined macro:
> #if __has_attribute(__fallthrough__)
> # define fallthrough                    __attribute__((__fallthrough__))
> #else
> # define fallthrough                    do {} while (0)  /* fallthrough */
> #endif
> 
> And it builds fine without the macro in the 'compat.h'.
> 
> 
> I added the define in the 'compat.h' for old kernels which doesn't
> define the macro. For that case I expect default '-Wimplicit-fallthrough'
> option is used which accepts 'fallthrough;' as a regex hit.

Your right, on older kernels it really is just a dummy statement.
The regex can't work, you can take (or leave the comment) it has no effect
because implicit-fallthrough won't see it in a macro.