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

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Message-ID: <CAOi1vP-bQR6aEbmyxUGtxTQACUbT2iEXr5Ld3w0p04_NWWxXuQ@mail.gmail.com>
Date:   Mon, 8 Oct 2018 18:00:19 +0200
From:   Ilya Dryomov <idryomov@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "Yan, Zheng" <zyan@...hat.com>, Sage Weil <sage@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>, pombredanne@...b.com,
        Luis Henriques <lhenriques@...e.com>,
        Ceph Development <ceph-devel@...r.kernel.org>,
        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 5:37 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> 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?

Fine with me.

Thanks,

                Ilya