linux-kernel - Re: [PATCH] ceph: use an enum instead of 'static const' to define constants

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Message-ID: <CAK8P3a0xpFdKC0AtBDC9UvMfwpx3HTbAdio4ApmYhrb2FakK9g@mail.gmail.com>
Date:   Mon, 8 Oct 2018 17:37:19 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Ilya Dryomov <idryomov@...il.com>
Cc:     Zheng Yan <zyan@...hat.com>, Sage Weil <sage@...hat.com>,
        gregkh <gregkh@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        lhenriques@...e.com, ceph-devel <ceph-devel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ceph: use an enum instead of 'static const' to define constants

On Mon, Oct 8, 2018 at 4:23 PM Ilya Dryomov <idryomov@...il.com> wrote:
> On Fri, Oct 5, 2018 at 6:18 PM Arnd Bergmann <arnd@...db.de> wrote:
> > @@ -71,7 +71,7 @@
> >   * This ensures that no two versions who have different meanings for
> >   * the bit ever speak to each other.
> >   */
> > -
> > +enum ceph_features {
> >  DEFINE_CEPH_FEATURE( 0, 1, UID)
> >  DEFINE_CEPH_FEATURE( 1, 1, NOSRCADDR)
> >  DEFINE_CEPH_FEATURE_RETIRED( 2, 1, MONCLOCKCHECK, JEWEL, LUMINOUS)
> > @@ -170,13 +170,13 @@ DEFINE_CEPH_FEATURE(61, 1, CEPHX_V2)             // *do not share this bit*
> >
> >  DEFINE_CEPH_FEATURE(62, 1, RESERVED)           // do not use; used as a sentinal
> >  DEFINE_CEPH_FEATURE_DEPRECATED(63, 1, RESERVED_BROKEN, LUMINOUS) // client-facing
> > -
> > +};
>
> I don't particularly like this because it looks like lower constants
> are actually ints and the rest are unsigned longs, even though they all
> have ULL suffixes.  The standard seems to require that enum constants
> be representable as ints, is the non-pedantic behaviour documented
> somewhere?

I had not realized that this is a gcc extension, or that it behaves slightly
differently from the standard C++ behavior that apparently adopted a
saner variant (all values in an enum have the same type).

How about we just add a __maybe_unused to DEFINE_CEPH_FEATURE
then to shut up the warning?

     Arnd