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Message-ID: <603f9243bb9e1c4c50aaec83a527266b48ab9e20.camel@perches.com>
Subject: Re: [PATCHv3 04/10] linux/kernel: introduce lower_48_bits macro
From:   Joe Perches <joe@perches.com>
To:     Keith Busch <kbusch@kernel.org>, Christoph Hellwig <hch@lst.de>
Cc:     linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
        linux-crypto@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, axboe@kernel.dk,
        martin.petersen@oracle.com, colyli@suse.de,
        Bart Van Assche <bvanassche@acm.org>
Date:   Tue, 22 Feb 2022 10:43:21 -0800
In-Reply-To: <20220222165613.GB1497257@dhcp-10-100-145-180.wdc.com>
References: <20220222163144.1782447-1-kbusch@kernel.org>
         <20220222163144.1782447-5-kbusch@kernel.org>
         <66a0c8210cf9e7dfcc3fa2d247de1eebd5a8acb7.camel@perches.com>
         <20220222165045.GA14168@lst.de>
         <20220222165613.GB1497257@dhcp-10-100-145-180.wdc.com>
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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.