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

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Message-ID: <Z9oiEkQEcHhA0a80@thinkpad>
Date: Tue, 18 Mar 2025 21:46:58 -0400
From: Yury Norov <yury.norov@...il.com>
To: linux-arm-kernel@...ts.infradead.org,
	Catalin Marinas <catalin.marinas@....com>
Cc: Lucas De Marchi <lucas.demarchi@...el.com>,
	Vincent Mailhol <mailhol.vincent@...adoo.fr>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Jani Nikula <jani.nikula@...ux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Tvrtko Ursulin <tursulin@...ulin.net>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org,
	Andi Shyti <andi.shyti@...ux.intel.com>,
	David Laight <David.Laight@...lab.com>,
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Jani Nikula <jani.nikula@...el.com>,
	Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v4 3/8] bits: introduce fixed-type genmasks

+ Catalin Marinas, ARM maillist

Hi Catalin and everyone,

Anshuman Khandual asked me to merge GENMASK_U128() saying it's
important for ARM to stabilize API. While it's a dead code, I
accepted his patch as he promised to add users shortly.

Now it's more than half a year since that. There's no users,
and no feedback from Anshuman.

Can you please tell if you still need the macro? I don't want to
undercut your development, but if you don't need 128-bit genmasks
there's no reason to have a dead code in the uapi.

Thanks,
Yury

On Wed, Mar 05, 2025 at 10:22:47AM -0500, Yury Norov wrote:
> + Anshuman Khandual <anshuman.khandual@....com>
> 
> Anshuman,
> 
> I merged your GENMASK_U128() because you said it's important for your
> projects, and that it will get used in the kernel soon.
> 
> Now it's in the kernel for more than 6 month, but no users were added.
> Can you clarify if you still need it, and if so why it's not used?
> 
> As you see, people add another fixed-types GENMASK() macros, and their
> implementation differ from GENMASK_U128().
> 
> My second concern is that __GENMASK_U128() is declared in uapi, while
> the general understanding for other fixed-type genmasks is that they
> are not exported to users. Do you need this macro to be exported to
> userspace? Can you show how and where it is used there?
> 
> Thanks,
> Yury
> 
> 
> On Wed, Mar 05, 2025 at 10:00:15PM +0900, Vincent Mailhol via B4 Relay wrote:
> > From: Yury Norov <yury.norov@...il.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@...il.com>
> > Signed-off-by: Lucas De Marchi <lucas.demarchi@...el.com>
> > Acked-by: Jani Nikula <jani.nikula@...el.com>
> > Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> > ---
> > 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
> >       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)				\
> > +	(GENMASK_INPUT_CHECK(h, l) +			\
> > +	 (((t)~ULL(0) - ((t)1 << (l)) + 1) &		\
> > +	  ((t)~ULL(0) >> (BITS_PER_TYPE(t) - 1 - (h)))))
> > +
> > +#define GENMASK(h, l) GENMASK_t(unsigned long,  h, l)
> > +#define GENMASK_ULL(h, l) GENMASK_t(unsigned long long, h, l)
> >  
> >  /*
> >   * Missing asm support
> >   *
> > + * __GENMASK_U*() depends on BITS_PER_TYPE() which would not work in the asm
> > + * 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))
> > +#define GENMASK_U32(h, l) GENMASK_t(u32, h, l)
> > +#define GENMASK_U64(h, l) GENMASK_t(u64, h, l)
> > +
> > +/*
> >   * __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
> > 
> > -- 
> > 2.45.3
> >