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

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Message-ID: <603f9243bb9e1c4c50aaec83a527266b48ab9e20.camel@perches.com>
Date:   Tue, 22 Feb 2022 10:43:21 -0800
From:   Joe Perches <joe@...ches.com>
To:     Keith Busch <kbusch@...nel.org>, Christoph Hellwig <hch@....de>
Cc:     linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, axboe@...nel.dk,
        martin.petersen@...cle.com, colyli@...e.de,
        Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCHv3 04/10] linux/kernel: introduce lower_48_bits macro

On Tue, 2022-02-22 at 08:56 -0800, Keith Busch wrote:
> On Tue, Feb 22, 2022 at 05:50:45PM +0100, Christoph Hellwig wrote:
> > 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:
> > > > +/ *
> > > > + * 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.
> 
> Sure, that sounds good to me. I only did it this way to match the
> existing local convention, but I personally prefer the inline function
> too. 

The existing convention is used there to allow the compiler to
avoid warnings and unnecessary conversions of a u32 to a u64 when
shifting by 32 or more bits.

If it's possible to be used with an architecture dependent typedef
like dma_addr_t, then perhaps it's reasonable to do something like:

#define lower_48_bits(val)					\
({								\
	typeof(val) high = lower_16_bits(upper_32_bits(val));	\
	typeof(val) low = lower_32_bits(val);			\
								\
	(high << 16 << 16) | low;				\
})

and have the compiler have the return value be an appropriate type.