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SubjectRe: [PATCH] checkpatch: Add --strict preference for #defines using BIT(foo)
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On Mon, 2014-11-10 at 15:36 -0800, Andrew Morton wrote:
> On Fri, 07 Nov 2014 13:15:39 -0800 Joe Perches <joe@perches.com> wrote:
>
> > Using BIT(foo) and BIT_ULL(bar) is more common now.
> > Suggest using these macros over #defines with 1<<value.
>
> urgh. I'm counting eightish implementations of BIT(), an unknown
> number of which are actually being used. Many use 1<<n, some use
> 1UL<<N, another uses 1ULL<<n. I'm a bit reluctant to recommend that
> anyone should use BIT() until it has has some vigorous scrubbing :(
>
> Is it actually an improvement? If I see
>
> #define X (1U << 7)
>
> then I know exactly what it does. Whereas when I see
>
> #define X BIT(7)
>
> I know neither the size or the signedness of X so I have to go look it
> up.

I'm not sure the signedness or type of X matters much
as the non-64bit comparisons are done by promotion to
at least int or unsigned int anyway.

The BIT macro makes sure a single bit is set.

The 'good' one is in bitops.h. It also has #define BIT_ULL

include/linux/bitops.h:#define BIT(nr) (1UL << (nr))
include/linux/bitops.h-#define BIT_ULL(nr) (1ULL << (nr))

The ones in tools/ are independent and should not be changed.
Excluding tools/, the others should probably be removed

$ git grep -E "define\s+BIT\b"
arch/arm/mach-davinci/sleep.S:#define BIT(nr) (1 << (nr))
drivers/staging/rtl8188eu/include/rtl8188e_spec.h:#define BIT(x) (1 << (x))
drivers/staging/rtl8188eu/include/wifi.h:#define BIT(x) (1 << (x))
drivers/staging/rtl8192e/rtl8192e/rtl_core.h:#define BIT(_i) (1<<(_i))
drivers/staging/rtl8712/osdep_service.h: #define BIT(x) (1 << (x))
drivers/staging/rtl8712/wifi.h:#define BIT(x) (1 << (x))

> I have no strong feelings either way, but I'm wondering what might have
> inspired this change?

David Miller commented on a netdev patch where 1<<foo was
being used in a #define and suggested using BIT.

http://article.gmane.org/gmane.linux.network/337535/match=bit

Using BIT _is_ more common in recent patches too.




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