linux-kernel - Re: [Intel-xe] [PATCH 2/3] linux/bits.h: Add fixed-width GENMASK and BIT macros

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Message-ID: <168364083689.27719.2337781022536351304@gjsousa-mobl2>
Date:   Tue, 9 May 2023 11:00:36 -0300
From:   Gustavo Sousa <gustavo.sousa@...el.com>
To:     Lucas De Marchi <lucas.demarchi@...el.com>,
        <dri-devel@...ts.freedesktop.org>,
        <intel-gfx@...ts.freedesktop.org>, <intel-xe@...ts.freedesktop.org>
CC:     Masahiro Yamada <masahiroy@...nel.org>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        <linux-kernel@...r.kernel.org>,
        Christian König <christian.koenig@....com>,
        Alex Deucher <alexander.deucher@....com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Intel-xe] [PATCH 2/3] linux/bits.h: Add fixed-width GENMASK and BIT macros

Quoting Lucas De Marchi (2023-05-09 02:14:02)
>Add GENMASK_U32(), GENMASK_U16() and GENMASK_U8()  macros to create
>masks for fixed-width types and also the corresponding BIT_U32(),
>BIT_U16() and BIT_U8().
>
>All of those depend on a new "U" suffix added to the integer constant.
>Due to naming clashes it's better to call the macro U32. Since C doesn't
>have a proper suffix for short and char types, the U16 and U18 variants
>just use U32 with one additional check in the BIT_* macros to make
>sure the compiler gives an error when the those types overflow.
>The BIT_U16() and BIT_U8() need the help of GENMASK_INPUT_CHECK(),
>as otherwise they would allow an invalid bit to be passed. Hence
>implement them in include/linux/bits.h rather than together with
>the other BIT* variants.
>
>The following test file is is used to test this:
>
>        $ cat mask.c
>        #include <linux/types.h>
>        #include <linux/bits.h>
>
>        static const u32 a = GENMASK_U32(31, 0);
>        static const u16 b = GENMASK_U16(15, 0);
>        static const u8 c = GENMASK_U8(7, 0);
>        static const u32 x = BIT_U32(31);
>        static const u16 y = BIT_U16(15);
>        static const u8 z = BIT_U8(7);
>
>        #if FAIL
>        static const u32 a2 = GENMASK_U32(32, 0);
>        static const u16 b2 = GENMASK_U16(16, 0);
>        static const u8 c2 = GENMASK_U8(8, 0);
>        static const u32 x2 = BIT_U32(32);
>        static const u16 y2 = BIT_U16(16);
>        static const u8 z2 = BIT_U8(8);
>        #endif
>
>Signed-off-by: Lucas De Marchi <lucas.demarchi@...el.com>
>---
> include/linux/bits.h       | 22 ++++++++++++++++++++++
> include/uapi/linux/const.h |  2 ++
> include/vdso/const.h       |  1 +
> 3 files changed, 25 insertions(+)
>
>diff --git a/include/linux/bits.h b/include/linux/bits.h
>index 7c0cf5031abe..ff4786c99b8c 100644
>--- a/include/linux/bits.h
>+++ b/include/linux/bits.h
>@@ -42,4 +42,26 @@
> #define GENMASK_ULL(h, l) \
>        (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))
> 
>+#define __GENMASK_U32(h, l) \
>+  (((~U32(0)) - (U32(1) << (l)) + 1) & \
>+   (~U32(0) >> (32 - 1 - (h))))
>+#define GENMASK_U32(h, l) \
>+  (GENMASK_INPUT_CHECK(h, l) + __GENMASK_U32(h, l))
>+
>+#define __GENMASK_U16(h, l) \
>+  ((U32(0xffff) - (U32(1) << (l)) + 1) & \
>+   (U32(0xffff) >> (16 - 1 - (h))))
>+#define GENMASK_U16(h, l) \
>+  (GENMASK_INPUT_CHECK(h, l) + __GENMASK_U16(h, l))
>+
>+#define __GENMASK_U8(h, l) \
>+  (((U32(0xff)) - (U32(1) << (l)) + 1) & \
>+   (U32(0xff) >> (8 - 1 - (h))))
>+#define GENMASK_U8(h, l) \
>+  (GENMASK_INPUT_CHECK(h, l) + __GENMASK_U8(h, l))

I wonder if we should use BIT_U* variants in the above to ensure the values are
valid. If so, we get a nice boundary check and we also can use a single
definition for the mask generation:

  #define __GENMASK_U32(h, l) \
          (((~U32(0)) - (U32(1) << (l)) + 1) & \
           (~U32(0) >> (32 - 1 - (h))))
  #define GENMASK_U32(h, l) \
          (GENMASK_INPUT_CHECK(h, l) + __GENMASK_U32(BIT_U32(h), BIT_U32(l)))
  #define GENMASK_U16(h, l) \
          (GENMASK_INPUT_CHECK(h, l) + __GENMASK_U32(BIT_U16(h), BIT_U16(l)))
  #define GENMASK_U8(h, l) \
          (GENMASK_INPUT_CHECK(h, l) + __GENMASK_U32(BIT_U8(h), BIT_U8(l)))

>+
>+#define BIT_U32(nr)       _BITU32(nr)
>+#define BIT_U16(nr)       (GENMASK_INPUT_CHECK(16 - 1, nr) + (U32(1) << (nr)))
>+#define BIT_U8(nr)        (GENMASK_INPUT_CHECK(32 - 1, nr) + (U32(1) << (nr)))

Shouldn't this be GENMASK_INPUT_CHECK(8 - 1, nr)?

--
Gustavo Sousa

>+
> #endif /* __LINUX_BITS_H */
>diff --git a/include/uapi/linux/const.h b/include/uapi/linux/const.h
>index a429381e7ca5..3a4e152520f4 100644
>--- a/include/uapi/linux/const.h
>+++ b/include/uapi/linux/const.h
>@@ -22,9 +22,11 @@
> #define _AT(T,X)       ((T)(X))
> #endif
> 
>+#define _U32(x)           (_AC(x, U))
> #define _UL(x)         (_AC(x, UL))
> #define _ULL(x)                (_AC(x, ULL))
> 
>+#define _BITU32(x)        (_U32(1) << (x))
> #define _BITUL(x)      (_UL(1) << (x))
> #define _BITULL(x)     (_ULL(1) << (x))
> 
>diff --git a/include/vdso/const.h b/include/vdso/const.h
>index 94b385ad438d..417384a9795b 100644
>--- a/include/vdso/const.h
>+++ b/include/vdso/const.h
>@@ -4,6 +4,7 @@
> 
> #include <uapi/linux/const.h>
> 
>+#define U32(x)            (_U32(x))
> #define UL(x)          (_UL(x))
> #define ULL(x)         (_ULL(x))
> 
>-- 
>2.40.1
>