linux-kernel - Re: [PATCH] kbuild: give SUBLEVEL more room in KERNEL

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Message-ID: <CAK7LNAStfNj0hvotctFonezQKQSbJfxU1HrwzWmyDc0+68fQ4Q@mail.gmail.com>
Date:   Mon, 18 Jan 2021 19:27:51 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Sasha Levin <sashal@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable@...nel.org
Subject: Re: [PATCH] kbuild: give SUBLEVEL more room in KERNEL_VERSION

On Mon, Jan 18, 2021 at 6:21 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Sun, Jan 17, 2021 at 08:49:51PM -0500, Sasha Levin wrote:
> > SUBLEVEL only has 8 bits of space, which means that we'll overflow it
> > once it reaches 256.
> >
> > Few of the stable branches will imminently overflow SUBLEVEL while
> > there's no risk of overflowing VERSION.
> >
> > Thus, give SUBLEVEL 8 more bits which will be stolen from VERSION, this
> > should create a better balance between the different version numbers we
> > use.
> >
> > The downside here is that Linus will have 8 bits less to play with, but
> > given our current release cadence (~10 weeks), the number of Linus's
> > fingers & toes (20), and the current VERSION (5) we can calculate that
> > VERSION will overflow in just over 1,000 years, so I'm kicking this can
> > down the road.
> >
> > Cc: stable@...nel.org
> > Signed-off-by: Sasha Levin <sashal@...nel.org>
> > ---
> >  Makefile | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 9e73f82e0d863..dc2bad7a440d8 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -1252,8 +1252,8 @@ endef
> >
> >  define filechk_version.h
> >       echo \#define LINUX_VERSION_CODE $(shell                         \
> > -     expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + 0$(SUBLEVEL)); \
> > -     echo '#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))'
> > +     expr $(VERSION) \* 16777216 + 0$(PATCHLEVEL) \* 65536 + 0$(SUBLEVEL)); \
> > +     echo '#define KERNEL_VERSION(a,b,c) (((a) << 24) + ((b) << 16) + (c))'
>
> As much as I agree, this will break in-tree users of LINUX_VERSION_CODE
> that try to suck out the version/patchlevel number of the kernel release
> into their own fields.  Things like USB host controller strings, v4l
> ioctl reports, scsi driver ioctls, and other places do fun bit-movements
> to try to unreverse this bit packing.


I can see a checkpatch warning about LINUX_VERSION_CODE.

See line 4528 of scripts/checkpatch.pl


  WARN("LINUX_VERSION_CODE",
       "LINUX_VERSION_CODE should be avoided, code should be for the
version to which it is merged\n" . $herecurr);



It helps external modules to be compiled for multiple kernel versions.

#if KERNEL_VERSION_CODE < KERNEL_VERSION(5, 4, 0)
    code for the kernel versions older than 5.4.0
#endif


The upstream code does not do this.
But, LINUX_VERSION_CODE is actually used in many places...





> So how about we just provide a "real" version/subversion/revision
> #define as well, and clean up all in-kernel users, so we can get this to
> work, and we can change it in the future more easily.
>
> thanks,
>
> greg k-h



-- 
Best Regards
Masahiro Yamada