linux-kernel - RE: [PATCHv3 04/10] linux/kernel: introduce lower_48

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Message-ID: <c5b76c7a0db647bd9850e6165cdb5da0@AcuMS.aculab.com>
Date:   Tue, 22 Feb 2022 17:09:47 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@....de>, Joe Perches <joe@...ches.com>
CC:     Keith Busch <kbusch@...nel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "colyli@...e.de" <colyli@...e.de>,
        "Bart Van Assche" <bvanassche@....org>
Subject: RE: [PATCHv3 04/10] linux/kernel: introduce lower_48_bits macro

From: Christoph Hellwig
> Sent: 22 February 2022 16:51
> 
> On Tue, Feb 22, 2022 at 08:45:53AM -0800, Joe Perches wrote:
> > On Tue, 2022-02-22 at 08:31 -0800, Keith Busch wrote:
> > > Recent data integrity field enhancements allow 48-bit reference tags.
> > > Introduce a helper macro since this will be a repeated operation.
> > []
> > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > []
> > > @@ -63,6 +63,12 @@
> > >  }					\
> > >  )
> > >
> > > +/**
> > > + * lower_48_bits - return bits 0-47 of a number
> > > + * @n: the number we're accessing
> > > + */
> > > +#define lower_48_bits(n) ((u64)((n) & 0xffffffffffffull))
> >
> > why not make this a static inline function?
> 
> Agreed.
> 
> > And visually, it's difficult to quickly count a repeated character to 12.
> >
> > Perhaps:
> >
> > static inline u64 lower_48_bits(u64 val)
> > {
> > 	return val & GENMASK_ULL(47, 0);
> > }
> 
> For anyone who has a minimum knowledge of C and hardware your version
> is an obsfucated clusterfuck, while the version Keith wrote is trivial
> to read.

I'd use the explicit: val & ((1ull << 48) - 1)
I think it is even fewer characters.

	David.

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