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References: <20240208074521.577076-1-lucas.demarchi@intel.com> <20240208074521.577076-2-lucas.demarchi@intel.com>
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From: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Date: Wed, 21 Feb 2024 22:30:02 +0200
Message-ID: <CAA8EJpprfrtOjNzT6TFhV1n6MXzLdTahanfxcRW4uVjeHaBduA@mail.gmail.com>
Subject: Re: [PATCH v3 1/3] bits: introduce fixed-type genmasks
To: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Yury Norov <yury.norov@gmail.com>, linux-kernel@vger.kernel.org, 
	dri-devel@lists.freedesktop.org, 
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>, 
	Jani Nikula <jani.nikula@linux.intel.com>, intel-xe@lists.freedesktop.org, 
	intel-gfx@lists.freedesktop.org, Jani Nikula <jani.nikula@intel.com>
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On Thu, 8 Feb 2024 at 09:45, Lucas De Marchi <lucas.demarchi@intel.com> wrote:
>
> From: Yury Norov <yury.norov@gmail.com>
>
> Generalize __GENMASK() to support different types, and implement
> fixed-types versions of GENMASK() based on it. The fixed-type version
> allows more strict checks to the min/max values accepted, which is
> useful for defining registers like implemented by i915 and xe drivers
> with their REG_GENMASK*() macros.
>
> The strict checks rely on shift-count-overflow compiler check to
> fail the build if a number outside of the range allowed is passed.
> Example:
>
>         #define FOO_MASK GENMASK_U32(33, 4)
>
> will generate a warning like:
>
>         ../include/linux/bits.h:41:31: error: left shift count >= width of type [-Werror=shift-count-overflow]
>            41 |          (((t)~0ULL - ((t)(1) << (l)) + 1) & \
>               |                               ^~
>
> Signed-off-by: Yury Norov <yury.norov@gmail.com>
> Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
> Acked-by: Jani Nikula <jani.nikula@intel.com>
> ---
>  include/linux/bitops.h |  1 -
>  include/linux/bits.h   | 32 ++++++++++++++++++++++----------
>  2 files changed, 22 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index 2ba557e067fe..1db50c69cfdb 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -15,7 +15,6 @@
>  #  define aligned_byte_mask(n) (~0xffUL << (BITS_PER_LONG - 8 - 8*(n)))
>  #endif
>
> -#define BITS_PER_TYPE(type)    (sizeof(type) * BITS_PER_BYTE)
>  #define BITS_TO_LONGS(nr)      __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
>  #define BITS_TO_U64(nr)                __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u64))
>  #define BITS_TO_U32(nr)                __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u32))
> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index 7c0cf5031abe..bd56f32de44e 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -6,6 +6,8 @@
>  #include <vdso/bits.h>
>  #include <asm/bitsperlong.h>
>
> +#define BITS_PER_TYPE(type)    (sizeof(type) * BITS_PER_BYTE)
> +
>  #define BIT_MASK(nr)           (UL(1) << ((nr) % BITS_PER_LONG))
>  #define BIT_WORD(nr)           ((nr) / BITS_PER_LONG)
>  #define BIT_ULL_MASK(nr)       (ULL(1) << ((nr) % BITS_PER_LONG_LONG))
> @@ -30,16 +32,26 @@
>  #define GENMASK_INPUT_CHECK(h, l) 0
>  #endif
>
> -#define __GENMASK(h, l) \
> -       (((~UL(0)) - (UL(1) << (l)) + 1) & \
> -        (~UL(0) >> (BITS_PER_LONG - 1 - (h))))
> -#define GENMASK(h, l) \
> -       (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> +/*
> + * Generate a mask for the specified type @t. Additional checks are made to
> + * guarantee the value returned fits in that type, relying on
> + * shift-count-overflow compiler check to detect incompatible arguments.
> + * For example, all these create build errors or warnings:
> + *
> + * - GENMASK(15, 20): wrong argument order
> + * - GENMASK(72, 15): doesn't fit unsigned long
> + * - GENMASK_U32(33, 15): doesn't fit in a u32
> + */
> +#define __GENMASK(t, h, l) \
> +       (GENMASK_INPUT_CHECK(h, l) + \
> +        (((t)~0ULL - ((t)(1) << (l)) + 1) & \
> +        ((t)~0ULL >> (BITS_PER_TYPE(t) - 1 - (h)))))
>
> -#define __GENMASK_ULL(h, l) \
> -       (((~ULL(0)) - (ULL(1) << (l)) + 1) & \
> -        (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h))))
> -#define GENMASK_ULL(h, l) \
> -       (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))
> +#define GENMASK(h, l)          __GENMASK(unsigned long,  h, l)
> +#define GENMASK_ULL(h, l)      __GENMASK(unsigned long long, h, l)
> +#define GENMASK_U8(h, l)       __GENMASK(u8,  h, l)
> +#define GENMASK_U16(h, l)      __GENMASK(u16, h, l)
> +#define GENMASK_U32(h, l)      __GENMASK(u32, h, l)
> +#define GENMASK_U64(h, l)      __GENMASK(u64, h, l)

This breaks drm-tip on arm64 architecture:

arch/arm64/kernel/entry-fpsimd.S: Assembler messages:
465arch/arm64/kernel/entry-fpsimd.S:271: Error: found 'l', expected: ')'
466arch/arm64/kernel/entry-fpsimd.S:66: Info: macro invoked from here
467arch/arm64/kernel/entry-fpsimd.S:271: Error: found 'l', expected: ')'
468arch/arm64/kernel/entry-fpsimd.S:66: Info: macro invoked from here
469arch/arm64/kernel/entry-fpsimd.S:271: Error: found 'l', expected: ')'
470arch/arm64/kernel/entry-fpsimd.S:66: Info: macro invoked from here
471arch/arm64/kernel/entry-fpsimd.S:271: Error: found 'l', expected: ')'
472arch/arm64/kernel/entry-fpsimd.S:66: Info: macro invoked from here
473arch/arm64/kernel/entry-fpsimd.S:271: Error: unexpected characters
following instruction at operand 3 -- `bic x2,x1,(0+(((unsigned
long)~0ULL-((unsigned long)(1)<<(0))+1)&((unsigned
long)~0ULL>>((sizeof(unsigned long)*8)-1-(3)))))'
474arch/arm64/kernel/entry-fpsimd.S:66: Info: macro invoked from here
475arch/arm64/kernel/entry-fpsimd.S:282: Error: found 'l', expected: ')'
476arch/arm64/kernel/entry-fpsimd.S:98: Info: macro invoked from here
477arch/arm64/kernel/entry-fpsimd.S:282: Error: found 'l', expected: ')'
478arch/arm64/kernel/entry-fpsimd.S:98: Info: macro invoked from here
479arch/arm64/kernel/entry-fpsimd.S:282: Error: found 'l', expected: ')'
480arch/arm64/kernel/entry-fpsimd.S:98: Info: macro invoked from here
481arch/arm64/kernel/entry-fpsimd.S:282: Error: found 'l', expected: ')'
482arch/arm64/kernel/entry-fpsimd.S:98: Info: macro invoked from here
483arch/arm64/kernel/entry-fpsimd.S:282: Error: unexpected characters
following instruction at operand 3 -- `bic x2,x1,(0+(((unsigned
long)~0ULL-((unsigned long)(1)<<(0))+1)&((unsigned
long)~0ULL>>((sizeof(unsigned long)*8)-1-(3)))))'
484arch/arm64/kernel/entry-fpsimd.S:98: Info: macro invoked from here

>
>  #endif /* __LINUX_BITS_H */
> --
> 2.43.0
>


-- 
With best wishes
Dmitry