linux-kernel - Re: [PATCH 1/2] linux/bits.h: make BIT(), GENMASK(), and friends available in assembly

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Date:   Tue, 11 Jun 2019 16:47:46 +0100
From:   Will Deacon <will.deacon@....com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH 1/2] linux/bits.h: make BIT(), GENMASK(), and friends
 available in assembly

On Mon, May 27, 2019 at 05:34:11PM +0900, Masahiro Yamada wrote:
> BIT(), GENMASK(), etc. are useful to define register bits of hardware.
> However, low-level code is often written in assembly, where they are
> not available due to the hard-coded 1UL, 0UL.
> 
> In fact, in-kernel headers such as arch/arm64/include/asm/sysreg.h
> use _BITUL() instead of BIT() so that the register bit macros are
> available in assembly.
> 
> Using macros in include/uapi/linux/const.h have two reasons:
> 
> [1] For use in uapi headers
>   We should use underscore-prefixed variants for user-space.
> 
> [2] For use in assembly code
>   Since _BITUL() does not use hard-coded 1UL, it can be used as an
>   alternative of BIT().
> 
> For [2], it is pretty easy to change BIT() etc. for use in assembly.
> 
> This allows to replace _BUTUL() in kernel headers with BIT().
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> ---
> 
>  include/linux/bits.h | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)

Acked-by: Will Deacon <will.deacon@....com>

Will