linux-kernel - [PATCH V3 1/2] uapi: Define GENMASK

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Message-Id: <20240801071646.682731-2-anshuman.khandual@arm.com>
Date: Thu,  1 Aug 2024 12:46:45 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: linux-kernel@...r.kernel.org
Cc: anshuman.khandual@....com,
	ardb@...nel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org
Subject: [PATCH V3 1/2] uapi: Define GENMASK_U128

This adds GENMASK_U128() and __GENMASK_U128() macros using __BITS_PER_U128
and __int128 data types. These macros will be used in providing support for
generating 128 bit masks.

Cc: Yury Norov <yury.norov@...il.com>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Arnd Bergmann <arnd@...db.de>>
Cc: linux-kernel@...r.kernel.org
Cc: linux-arch@...r.kernel.org
Reviewed-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
---
 include/linux/bits.h       | 13 +++++++++++++
 include/uapi/linux/bits.h  |  3 +++
 include/uapi/linux/const.h | 15 +++++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/include/linux/bits.h b/include/linux/bits.h
index 0eb24d21aac2..bf99feb5570e 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -36,4 +36,17 @@
 #define GENMASK_ULL(h, l) \
 	(GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))
 
+/*
+ * Missing asm support
+ *
+ * __GENMASK_U128() depends on _BIT128() which would not work
+ * in the asm code, as it shifts an 'unsigned __init128' 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.
+ */
+#define GENMASK_U128(h, l) \
+	(GENMASK_INPUT_CHECK(h, l) + __GENMASK_U128(h, l))
+
 #endif	/* __LINUX_BITS_H */
diff --git a/include/uapi/linux/bits.h b/include/uapi/linux/bits.h
index 3c2a101986a3..4d4b7b08003c 100644
--- a/include/uapi/linux/bits.h
+++ b/include/uapi/linux/bits.h
@@ -12,4 +12,7 @@
         (((~_ULL(0)) - (_ULL(1) << (l)) + 1) & \
          (~_ULL(0) >> (__BITS_PER_LONG_LONG - 1 - (h))))
 
+#define __GENMASK_U128(h, l) \
+	((_BIT128((h) + 1)) - (_BIT128(l)))
+
 #endif /* _UAPI_LINUX_BITS_H */
diff --git a/include/uapi/linux/const.h b/include/uapi/linux/const.h
index a429381e7ca5..5be12e8f8f9c 100644
--- a/include/uapi/linux/const.h
+++ b/include/uapi/linux/const.h
@@ -28,6 +28,21 @@
 #define _BITUL(x)	(_UL(1) << (x))
 #define _BITULL(x)	(_ULL(1) << (x))
 
+/*
+ * Missing asm support
+ *
+ * __BIT128() would not work in the asm code, as it shifts an
+ * 'unsigned __init128' data type as direct representation of
+ * 128 bit constants is not supported in the gcc compiler, as
+ * they get silently truncated.
+ *
+ * TODO: Please revisit this implementation when gcc compiler
+ * starts representing 128 bit constants directly like long
+ * and unsigned long etc. Subsequently drop the comment for
+ * GENMASK_U128() which would then start supporting asm code.
+ */
+#define _BIT128(x)	((unsigned __int128)(1) << (x))
+
 #define __ALIGN_KERNEL(x, a)		__ALIGN_KERNEL_MASK(x, (__typeof__(x))(a) - 1)
 #define __ALIGN_KERNEL_MASK(x, mask)	(((x) + (mask)) & ~(mask))
 
-- 
2.30.2