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Message-ID: <1b5f5121-67d8-49ca-94ef-8ac24aa90424@ghiti.fr>
Date: Fri, 18 Jul 2025 15:03:46 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Ignacio Encinas <ignacio@...cinas.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Cc: linux-kernel-mentees@...ts.linux.dev, skhan@...uxfoundation.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Palmer Dabbelt <palmer@...osinc.com>
Subject: Re: [PATCH v5] riscv: introduce asm/swab.h

Hi Ignacio,

On 7/17/25 20:44, Ignacio Encinas wrote:
> Implement endianness swap macros for RISC-V.
>
> Use the rev8 instruction when Zbb is available. Otherwise, rely on the
> default mask-and-shift implementation.
>
> Acked-by: Palmer Dabbelt <palmer@...osinc.com>
> Signed-off-by: Ignacio Encinas <ignacio@...cinas.com>
> ---
> Motivated by [1]. Tested with crc_kunit as pointed out here [2]. I can't
> provide performance numbers as I don't have RISC-V hardware.
>
> [1] https://lore.kernel.org/all/20250302220426.GC2079@quark.localdomain/
> [2] https://lore.kernel.org/all/20250216225530.306980-1-ebiggers@kernel.org/
> ---
> Changes in v5:
> - Duplicate ___constant_swab helpers in arch/riscv/include/asm/swab.h to
>    avoid delaying the patch as suggested by Alex in [3] (drop patch 1 and
>    convert this into a 1-patch series)
> - Link to v4: https://lore.kernel.org/r/20250426-riscv-swab-v4-0-64201404a68c@iencinas.com
>
> [3] https://lore.kernel.org/linux-riscv/7e22a448-3cee-4475-b69b-3dd45b57f168@ghiti.fr/
>
> Changes in v4:
>
> - Add missing include in the 1st patch, reported by
>    https://lore.kernel.org/all/202504042300.it9RcOSt-lkp@intel.com/
> - Rewrite the ARCH_SWAB macro as suggested by Arnd
> - Define __arch_swab64 for CONFIG_32BIT (Ben)
> - Link to v3: https://lore.kernel.org/r/20250403-riscv-swab-v3-0-3bf705d80e33@iencinas.com
>
> Changes in v3:
>
> PATCH 2:
>    Use if(riscv_has_extension_likely) instead of asm goto (Eric). It
>    looks like both versions generate the same assembly. Perhaps we should
>    do the same change in other places such as arch/riscv/include/asm/bitops.h
> - Link to v2: https://lore.kernel.org/r/20250319-riscv-swab-v2-0-d53b6d6ab915@iencinas.com
>
> Changes in v2:
> - Introduce first patch factoring out the default implementation into
>    asm-generic
> - Remove blank line to make checkpatch happy
> - Link to v1: https://lore.kernel.org/r/20250310-riscv-swab-v1-1-34652ef1ee96@iencinas.com
> ---
>   arch/riscv/include/asm/swab.h | 87 +++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 87 insertions(+)
>
> diff --git a/arch/riscv/include/asm/swab.h b/arch/riscv/include/asm/swab.h
> new file mode 100644
> index 0000000000000000000000000000000000000000..4f408f59fada7251d62f56d174ae76ff19f4a319
> --- /dev/null
> +++ b/arch/riscv/include/asm/swab.h
> @@ -0,0 +1,87 @@
> +/* 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/cpufeature-macros.h>
> +#include <asm/hwcap.h>
> +#include <asm-generic/swab.h>
> +
> +#if defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE)
> +
> +// Duplicated from include/uapi/linux/swab.h
> +#define ___constant_swab16(x) ((__u16)(				\
> +	(((__u16)(x) & (__u16)0x00ffU) << 8) |			\
> +	(((__u16)(x) & (__u16)0xff00U) >> 8)))
> +
> +#define ___constant_swab32(x) ((__u32)(				\
> +	(((__u32)(x) & (__u32)0x000000ffUL) << 24) |		\
> +	(((__u32)(x) & (__u32)0x0000ff00UL) <<  8) |		\
> +	(((__u32)(x) & (__u32)0x00ff0000UL) >>  8) |		\
> +	(((__u32)(x) & (__u32)0xff000000UL) >> 24)))
> +
> +#define ___constant_swab64(x) ((__u64)(				\
> +	(((__u64)(x) & (__u64)0x00000000000000ffULL) << 56) |	\
> +	(((__u64)(x) & (__u64)0x000000000000ff00ULL) << 40) |	\
> +	(((__u64)(x) & (__u64)0x0000000000ff0000ULL) << 24) |	\
> +	(((__u64)(x) & (__u64)0x00000000ff000000ULL) <<  8) |	\
> +	(((__u64)(x) & (__u64)0x000000ff00000000ULL) >>  8) |	\
> +	(((__u64)(x) & (__u64)0x0000ff0000000000ULL) >> 24) |	\
> +	(((__u64)(x) & (__u64)0x00ff000000000000ULL) >> 40) |	\
> +	(((__u64)(x) & (__u64)0xff00000000000000ULL) >> 56)))
> +
> +#define ARCH_SWAB(size, value)						\
> +({									\
> +	unsigned long x = value;					\
> +									\
> +	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));			\
> +		x = x >> (BITS_PER_LONG - size);			\
> +	} else {                                                        \
> +		x = ___constant_swab##size(value);                      \
> +	}								\
> +	x;								\
> +})
> +
> +static __always_inline __u16 __arch_swab16(__u16 value)
> +{
> +	return ARCH_SWAB(16, value);
> +}
> +
> +static __always_inline __u32 __arch_swab32(__u32 value)
> +{
> +	return ARCH_SWAB(32, value);
> +}
> +
> +#ifdef CONFIG_64BIT
> +static __always_inline __u64 __arch_swab64(__u64 value)
> +{
> +	return ARCH_SWAB(64, value);
> +}
> +#else
> +static __always_inline __u64 __arch_swab64(__u64 value)
> +{
> +	__u32 h = value >> 32;
> +	__u32 l = value & ((1ULL << 32) - 1);
> +
> +	return ((__u64)(__arch_swab32(l)) << 32) | ((__u64)(__arch_swab32(h)));
> +}
> +#endif
> +
> +#define __arch_swab64 __arch_swab64
> +#define __arch_swab32 __arch_swab32
> +#define __arch_swab16 __arch_swab16
> +
> +#undef ___constant_swab16
> +#undef ___constant_swab32
> +#undef ___constant_swab64
> +
> +#undef ARCH_SWAB
> +
> +#endif /* defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE) */
> +#endif /* _ASM_RISCV_SWAB_H */
>
> ---
> base-commit: 155a3c003e555a7300d156a5252c004c392ec6b0
> change-id: 20250307-riscv-swab-b81b94a9ac1b
>
> Best regards,


Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>
Tested-by: Alexandre Ghiti <alexghiti@...osinc.com>

Thanks for the quick new version!

Alex


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