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Subject: Re: [PATCHv3 04/10] linux/kernel: introduce lower_48_bits macro
From:   Joe Perches <joe@perches.com>
To:     David Laight <David.Laight@ACULAB.COM>,
        Keith Busch <kbusch@kernel.org>, Christoph Hellwig <hch@lst.de>
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        "colyli@suse.de" <colyli@suse.de>,
        Bart Van Assche <bvanassche@acm.org>
Date:   Tue, 22 Feb 2022 12:31:30 -0800
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On Tue, 2022-02-22 at 20:09 +0000, David Laight wrote:
> From: Joe Perches
> > Sent: 22 February 2022 18:43
> > 
> > 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.
> 
> The compiler could make a real pigs breakfast of that.

Both gcc and clang optimize it just fine.

btw: to return the same type the last line should be:

	(typeof(val))((high << 16 << 16) | low);

otherwise the return is sizeof(int) if typeof(val) is not u64

> Oh, did you look for GENMASK([^,]*,[ 0]*) ?

No, why?  I did look for 47, 0 though.

But it's pretty common really.

$ git grep -P 'GENMASK(?:_ULL)?\s*\(\s*\d+\s*,\s*0\s*\)' | wc -l
6233

> I'd only use something GENMASK() for bit ranges.
> Even then it is often easier to just write the value in hex.

Mostly it's the count of the repeated f that's difficult to
quickly verify visually.

> I think the only time I've written anything like that recently
> (last 30 years) was for some hardware registers when the documentation
> user 'bit 1' for the most significant bit.

Luckily the hardware I've had to deal with never did that.
Not even the least significant bit too.