linux-kernel - Re: [RFC 01/32] fs: introduce new 'struct inode

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Date:	Sat, 31 May 2014 16:54:41 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Andreas Schwab <schwab@...ux-m68k.org>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"Joseph S. Myers" <joseph@...esourcery.com>,
	John Stultz <john.stultz@...aro.org>,
	Christoph Hellwig <hch@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ley Foon Tan <lftan@...era.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC 01/32] fs: introduce new 'struct inode_time'

On Saturday 31 May 2014 10:39:02 Andreas Schwab wrote:
> Geert Uytterhoeven <geert@...ux-m68k.org> writes:
> 
> > Hi Arnd,
> >
> > On Fri, May 30, 2014 at 10:01 PM, Arnd Bergmann <arnd@...db.de> wrote:
> >> + * The variant using bit fields is less efficient to access, but
> >> + * small and has a wider range as the 32-bit one, plus it keeps
> >> + * the signedness of the original timespec.
> >> + */
> >> +struct inode_time {
> >> +       long long       tv_sec  : 34;
> >> +       int             tv_nsec : 30;
> >> +};
> >
> > Don't you need 31 bits for tv_nsec, to accommodate for the sign bit?
> > I know you won't really store negative numbers there, but storing a large
> > positive number will become negative on read out, won't it?
> 
> Only if the int bitfield is signed.  Bitfields are weird, aren't they? 

It was a mistake on my side, as I didn't know about that rule and
meant write 'unsigned int' really. Also, I always have a bad feeling
about using bitfields in general.

	Arnd
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