linux-kernel - [PATCH v2 2/2] riscv: introduce asm/swab.h

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Message-Id: <20250319-riscv-swab-v2-2-d53b6d6ab915@iencinas.com>
Date: Wed, 19 Mar 2025 22:09:46 +0100
From: Ignacio Encinas <ignacio@...cinas.com>
To: Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Alexandre Ghiti <alex@...ti.fr>, 
 Arnd Bergmann <arnd@...db.de>
Cc: Eric Biggers <ebiggers@...nel.org>, linux-riscv@...ts.infradead.org, 
 linux-kernel@...r.kernel.org, linux-kernel-mentees@...ts.linux.dev, 
 skhan@...uxfoundation.org, Zhihang Shao <zhihang.shao.iscas@...il.com>, 
 Björn Töpel <bjorn@...nel.org>, 
 linux-arch@...r.kernel.org, Ignacio Encinas <ignacio@...cinas.com>
Subject: [PATCH v2 2/2] riscv: introduce asm/swab.h

Implement endianness swap macros for RISC-V.

Use the rev8 instruction when Zbb is available. Otherwise, rely on the
default mask-and-shift implementation.

Signed-off-by: Ignacio Encinas <ignacio@...cinas.com>
---
 arch/riscv/include/asm/swab.h | 48 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/arch/riscv/include/asm/swab.h b/arch/riscv/include/asm/swab.h
new file mode 100644
index 0000000000000000000000000000000000000000..6cb40e8108c956dd445746d59bc1dd0a53475212
--- /dev/null
+++ b/arch/riscv/include/asm/swab.h
@@ -0,0 +1,48 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _ASM_RISCV_SWAB_H
+#define _ASM_RISCV_SWAB_H
+
+#include <linux/types.h>
+#include <linux/compiler.h>
+#include <asm/alternative-macros.h>
+#include <asm/hwcap.h>
+#include <asm-generic/swab.h>
+
+#if defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE)
+
+#define ARCH_SWAB(size) \
+static __always_inline unsigned long __arch_swab##size(__u##size value) \
+{									\
+	unsigned long x = value;					\
+									\
+	asm goto(ALTERNATIVE("j %l[legacy]", "nop", 0,			\
+			     RISCV_ISA_EXT_ZBB, 1)			\
+			     :::: legacy);				\
+									\
+	asm volatile (".option push\n"					\
+		      ".option arch,+zbb\n"				\
+		      "rev8 %0, %1\n"					\
+		      ".option pop\n"					\
+		      : "=r" (x) : "r" (x));				\
+									\
+	return x >> (BITS_PER_LONG - size);				\
+									\
+legacy:									\
+	return  ___constant_swab##size(value);				\
+}
+
+#ifdef CONFIG_64BIT
+ARCH_SWAB(64)
+#define __arch_swab64 __arch_swab64
+#endif
+
+ARCH_SWAB(32)
+#define __arch_swab32 __arch_swab32
+
+ARCH_SWAB(16)
+#define __arch_swab16 __arch_swab16
+
+#undef ARCH_SWAB
+
+#endif /* defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE) */
+#endif /* _ASM_RISCV_SWAB_H */

-- 
2.48.1