Re: [PATCH v4 3/8] bits: introduce fixed-type genmasks

Yury Norov
2025-03-05T15:47:04Z

On Wed, Mar 05, 2025 at 10:00:15PM +0900, Vincent Mailhol via B4 Relay wrote:
> From: Yury Norov <yury.norov@gmail.com>
>
> Add __GENMASK_t() which generalizes __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>
> Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>

Co-developed-by?

> ---
> Changelog:
>
> v3 -> v4:
>
> - The v3 is one year old. Meanwhile people started using
> __GENMASK() directly. So instead of generalizing __GENMASK() to
> support different types, add a new GENMASK_t().
>
> - replace ~0ULL by ~_ULL(0). Otherwise, __GENMASK_t() would fail
> in asm code.
>
> - Make GENMASK_U8() and GENMASK_U16() return an unsigned int. In
> v3, due to the integer promotion rules, these were returning a
> signed integer. By casting these to unsigned int, at least the

This comment will disappear when I'll apply the patch. Can you comment
it in the code instead?

> signedness is kept.
> ---
> include/linux/bitops.h | 1 -
> include/linux/bits.h | 33 +++++++++++++++++++++++++++++----
> 2 files changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index c1cb53cf2f0f8662ed3e324578f74330e63f935d..9be2d50da09a417966b3d11c84092bb2f4cd0bef 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -8,7 +8,6 @@
>
> #include <uapi/linux/kernel.h>
>
> -#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 5f68980a1b98d771426872c74d7b5c0f79e5e802..f202e46d2f4b7899c16d975120f3fa3ae41556ae 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -12,6 +12,7 @@
> #define BIT_ULL_MASK(nr) (ULL(1) << ((nr) % BITS_PER_LONG_LONG))
> #define BIT_ULL_WORD(nr) ((nr) / BITS_PER_LONG_LONG)
> #define BITS_PER_BYTE 8
> +#define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
>
> /*
> * Create a contiguous bitmask starting at bit position @l and ending at
> @@ -25,14 +26,38 @@
>
> #define GENMASK_INPUT_CHECK(h, l) BUILD_BUG_ON_ZERO(const_true((l) > (h)))
>
> -#define GENMASK(h, l) \
> - (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> -#define GENMASK_ULL(h, l) \
> - (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(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(t, h, l) \

Agree with Andy. This should be GENMASK_TYPE, or triple-underscored
___GENMASK() maybe. This _t thing looks misleading.

> + (GENMASK_INPUT_CHECK(h, l) + \
> + (((t)~ULL(0) - ((t)1 << (l)) + 1) & \
> + ((t)~ULL(0) >> (BITS_PER_TYPE(t) - 1 - (h)))))

Can you rebase it on top of -next? In this dev cycle I merge a patch
that reverts the __GENMASK() back to:

#define __GENMASK(h, l) (((~_UL(0)) << (l)) & (~_UL(0) >> (BITS_PER_LONG - 1 - (h))))

> +#define GENMASK(h, l) GENMASK_t(unsigned long, h, l)
> +#define GENMASK_ULL(h, l) GENMASK_t(unsigned long long, h, l)

This makes __GENMASK() and __GENMASK_ULL() unused in the kernel, other
than in uapi. Or I misunderstand it?

Having, in fact, different implementations of the same macro for kernel
and userspace is a source of problems. Can we move GENMASK_TYPE() to uapi,
and implement __GENMASK() on top of them? If not, I'd prefer to keep
GENMASK and GENMASK_ULL untouched.

Can you run bloat-o-meter and ensure there's no unwanted effects on
code generation?

> /*
> * Missing asm support
> *
> + * __GENMASK_U*() depends on BITS_PER_TYPE() which would not work in the asm

And there's no __GENMASK_U*(), right?

> + * code as BITS_PER_TYPE() relies on sizeof(), something not available in
> + * asm. Nethertheless, the concept of fixed width integers is a C thing which
> + * does not apply to assembly code.
> + */
> +#define GENMASK_U8(h, l) ((unsigned int)GENMASK_t(u8, h, l))
> +#define GENMASK_U16(h, l) ((unsigned int)GENMASK_t(u16, h, l))

Typecast to the type that user provides explicitly? And maybe do
in GENMASK_TYPE()

> +#define GENMASK_U32(h, l) GENMASK_t(u32, h, l)
> +#define GENMASK_U64(h, l) GENMASK_t(u64, h, l)

OK, this looks good. But GENMASK_U128() becomes a special case now.
The 128-bit GENMASK is unsued, but it's exported in uapi. Is there any
simple way to end up with a common implementation for all fixed-type
GENMASKs?

> +
> +/*
> * __GENMASK_U128() depends on _BIT128() which would not work
> * in the asm code, as it shifts an 'unsigned __int128' data
> * type instead of direct representation of 128 bit constants

This comment is duplicated by the previous one. Maybe just join them?
(Let's wait for a while for updates regarding GENMASK_U128 status before
doing it.)