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@iencinas.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

On Wed, Mar 19, 2025 at 10:09:46PM +0100, Ignacio Encinas wrote:
> +#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);				\

Is there a reason to use this instead of
riscv_has_extension_likely(RISCV_ISA_EXT_ZBB) which seems to do the same thing,
including using a static branch?

- Eric

On 21/3/25 4:37, Eric Biggers wrote:
> On Wed, Mar 19, 2025 at 10:09:46PM +0100, Ignacio Encinas wrote:
>> +#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);				\
> 
> Is there a reason to use this instead of
> riscv_has_extension_likely(RISCV_ISA_EXT_ZBB) which seems to do the same thing,
> including using a static branch?

I just followed what's already in arch/riscv/include/asm/bitops.h

However, I changed it to

	if(riscv_has_extension_likely(RISCV_ISA_EXT_ZBB)) {
		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);
	}

	return  ___constant_swab##size(value);

and it seems gcc generates the exact same code. I tested it with 
arch/riscv/lib/csum.c (which uses swab32) and both versions generate the
exact same object file.

This certainly looks easier to read. If there are no complaints I'll
send a v3 using a plain if with riscv_has_extension_likely.

Thanks for pointing it out!