linux-kernel - [PATCH v5 2/7] bits: introduce fixed-type genmasks

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Message-Id: <20250306-fixed-type-genmasks-v5-2-b443e9dcba63@wanadoo.fr>
Date: Thu, 06 Mar 2025 20:29:53 +0900
From: Vincent Mailhol via B4 Relay <devnull+mailhol.vincent.wanadoo.fr@...nel.org>
To: Yury Norov <yury.norov@...il.com>, 
 Lucas De Marchi <lucas.demarchi@...el.com>, 
 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>
Cc: 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>, 
 Vincent Mailhol <mailhol.vincent@...adoo.fr>
Subject: [PATCH v5 2/7] bits: introduce fixed-type genmasks

From: Yury Norov <yury.norov@...il.com>

Add GENMASK_TYPE() 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:51:27: error: right shift count >= width of type [-Werror=shift-count-overflow]
     51 |               type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))
        |                           ^~

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>
Co-developed-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
---
Changelog:

  v4 -> v5:

    - Rename GENMASK_t() to GENMASK_TYPE().

    - Fix typo in patch description.

    - Use tab indentations instead of single space to separate the
      macro name from its body.

    - s/__GENMASK_U*()/GENMASK_U*()/g in the comment.

    - Add a tag to credit myself as Co-developer. Keep Yury as the
      main author.

    - Modify GENMASK_TYPE() to match the changes made to __GENMASK()
      in: https://github.com/norov/linux/commit/1e7933a575ed

    - Replace (t)~_ULL(0) with type_max(t). This is OK because
      GENMASK_TYPE() is not available in asm.

    - linux/const.h and asm/bitsperlong.h are not used anymore. Remove
      them.

    - Apply GENMASK_TYPE() to GENMASK_U128().

    - Remove the unsigned int cast for the U8 and U16 variants. Cast
      to the target type instead. Do that cast directly in
      GENMASK_TYPE().

  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   | 47 +++++++++++++++++++++++++++++++----------------
 2 files changed, 31 insertions(+), 17 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 4819cbe7bd48fbae796fc6005c9f92b1c93a034c..74219521a56e2639ccff7fdc899d6805ee355a0c 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -2,16 +2,15 @@
 #ifndef __LINUX_BITS_H
 #define __LINUX_BITS_H
 
-#include <linux/const.h>
 #include <vdso/bits.h>
 #include <uapi/linux/bits.h>
-#include <asm/bitsperlong.h>
 
 #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))
 #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
@@ -20,28 +19,44 @@
  */
 #if !defined(__ASSEMBLY__)
 
+/*
+ * Missing asm support
+ *
+ * GENMASK_U*() depends on BITS_PER_TYPE() which relies on sizeof(),
+ * something not available in asm. Nethertheless, fixed width integers
+ * is a C concept. Assembly code can rely on the long and long long
+ * versions instead.
+ */
+
 #include <linux/build_bug.h>
 #include <linux/compiler.h>
+#include <linux/overflow.h>
 
 #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))
-
 /*
- * Missing asm support
+ * 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_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
- * such as long and unsigned long. The fundamental problem is
- * that a 128 bit constant will get silently truncated by the
- * gcc compiler.
+ * - 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_U128(h, l) \
-	(GENMASK_INPUT_CHECK(h, l) + __GENMASK_U128(h, l))
+#define GENMASK_TYPE(t, h, l)					\
+	((t)(GENMASK_INPUT_CHECK(h, l) +			\
+	     (type_max(t) << (l) &				\
+	      type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))
+
+#define GENMASK(h, l)		GENMASK_TYPE(unsigned long,  h, l)
+#define GENMASK_ULL(h, l)	GENMASK_TYPE(unsigned long long, h, l)
+
+#define GENMASK_U8(h, l)	GENMASK_TYPE(u8,  h, l)
+#define GENMASK_U16(h, l)	GENMASK_TYPE(u16, h, l)
+#define GENMASK_U32(h, l)	GENMASK_TYPE(u32, h, l)
+#define GENMASK_U64(h, l)	GENMASK_TYPE(u64, h, l)
+#define GENMASK_U128(h, l)	GENMASK_TYPE(u128, h, l)
 
 #else /* defined(__ASSEMBLY__) */
 

-- 
2.45.3