Re: [PATCHv3 04/10] linux/kernel: introduce lower_48_bits macro

Joe Perches
2022-02-22T18:43:28Z

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.