linux-kernel - Re: [RESEND PATCH] arm64: v8.4: Support for new floating point multiplication variant

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Message-ID: <20171211115947.GS12608@e103592.cambridge.arm.com>
Date:   Mon, 11 Dec 2017 11:59:47 +0000
From:   Dave P Martin <Dave.Martin@....com>
To:     Dongjiu Geng <gengdongjiu@...wei.com>
Cc:     Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <Will.Deacon@....com>,
        "corbet@....net" <corbet@....net>,
        Mark Rutland <Mark.Rutland@....com>,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        Robin Murphy <Robin.Murphy@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "arvind.yadav.cs@...il.com" <arvind.yadav.cs@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>,
        "huangshaoyu@...wei.com" <huangshaoyu@...wei.com>,
        "guohanjun@...wei.com" <guohanjun@...wei.com>,
        "zhanghaibin7@...wei.com" <zhanghaibin7@...wei.com>,
        "zhihui.gao@...wei.com" <zhihui.gao@...wei.com>
Subject: Re: [RESEND PATCH] arm64: v8.4: Support for new floating point
 multiplication variant

On Sat, Dec 09, 2017 at 03:28:42PM +0000, Dongjiu Geng wrote:
> ARM v8.4 extensions include support for new floating point
> multiplication variant instructions to the AArch64 SIMD

Do we have any human-readable description of what the new instructions
do?

Since the v8.4 spec itself only describes these as "New Floating
Point Multiplication Variant", I wonder what "FHM" actually stands
for.

Maybe something like "widening half-precision floating-point multiply
accumulate" is acceptable wording consistent with the existing
architecture, but I just made that up, so it's not official ;)

> instructions set. Let the userspace know about it via a
> HWCAP bit and MRS emulation.
>
> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> Signed-off-by: Dongjiu Geng <gengdongjiu@...wei.com>
> ---
> My platform supports this feature, so I need to add it.
> ---
>  Documentation/arm64/cpu-feature-registers.txt | 4 +++-
>  arch/arm64/include/asm/sysreg.h               | 1 +
>  arch/arm64/include/uapi/asm/hwcap.h           | 1 +
>  arch/arm64/kernel/cpufeature.c                | 2 ++
>  arch/arm64/kernel/cpuinfo.c                   | 1 +
>  5 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/arm64/cpu-feature-registers.txt b/Documentation/arm64/cpu-feature-registers.txt
> index bd9b3fa..a70090b 100644
> --- a/Documentation/arm64/cpu-feature-registers.txt
> +++ b/Documentation/arm64/cpu-feature-registers.txt
> @@ -110,7 +110,9 @@ infrastructure:
>       x--------------------------------------------------x
>       | Name                         |  bits   | visible |
>       |--------------------------------------------------|
> -     | RES0                         | [63-48] |    n    |
> +     | RES0                         | [63-52] |    n    |
> +     |--------------------------------------------------|
> +     | FHM                          | [51-48] |    y    |

You also need to update Documentation/arm64/elf_hwcaps.txt.

Otherwise, looks OK.

Cheers
---Dave

>       |--------------------------------------------------|
>       | DP                           | [47-44] |    y    |
>       |--------------------------------------------------|
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 08cc885..1818077 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -419,6 +419,7 @@
>  #define SCTLR_EL1_CP15BEN    (1 << 5)
>
>  /* id_aa64isar0 */
> +#define ID_AA64ISAR0_FHM_SHIFT               48
>  #define ID_AA64ISAR0_DP_SHIFT                44
>  #define ID_AA64ISAR0_SM4_SHIFT               40
>  #define ID_AA64ISAR0_SM3_SHIFT               36
> diff --git a/arch/arm64/include/uapi/asm/hwcap.h b/arch/arm64/include/uapi/asm/hwcap.h
> index cda76fa..f018c3d 100644
> --- a/arch/arm64/include/uapi/asm/hwcap.h
> +++ b/arch/arm64/include/uapi/asm/hwcap.h
> @@ -43,5 +43,6 @@
>  #define HWCAP_ASIMDDP                (1 << 20)
>  #define HWCAP_SHA512         (1 << 21)
>  #define HWCAP_SVE            (1 << 22)
> +#define HWCAP_ASIMDFHM               (1 << 23)
>
>  #endif /* _UAPI__ASM_HWCAP_H */
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index c5ba009..bc7e707 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -123,6 +123,7 @@ static int __init register_cpu_hwcaps_dumper(void)
>   * sync with the documentation of the CPU feature register ABI.
>   */
>  static const struct arm64_ftr_bits ftr_id_aa64isar0[] = {
> +     ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR0_FHM_SHIFT, 4, 0),
>       ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR0_DP_SHIFT, 4, 0),
>       ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR0_SM4_SHIFT, 4, 0),
>       ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR0_SM3_SHIFT, 4, 0),
> @@ -991,6 +992,7 @@ static bool has_no_fpsimd(const struct arm64_cpu_capabilities *entry, int __unus
>       HWCAP_CAP(SYS_ID_AA64ISAR0_EL1, ID_AA64ISAR0_SM3_SHIFT, FTR_UNSIGNED, 1, CAP_HWCAP, HWCAP_SM3),
>       HWCAP_CAP(SYS_ID_AA64ISAR0_EL1, ID_AA64ISAR0_SM4_SHIFT, FTR_UNSIGNED, 1, CAP_HWCAP, HWCAP_SM4),
>       HWCAP_CAP(SYS_ID_AA64ISAR0_EL1, ID_AA64ISAR0_DP_SHIFT, FTR_UNSIGNED, 1, CAP_HWCAP, HWCAP_ASIMDDP),
> +     HWCAP_CAP(SYS_ID_AA64ISAR0_EL1, ID_AA64ISAR0_FHM_SHIFT, FTR_UNSIGNED, 1, CAP_HWCAP, HWCAP_ASIMDFHM),
>       HWCAP_CAP(SYS_ID_AA64PFR0_EL1, ID_AA64PFR0_FP_SHIFT, FTR_SIGNED, 0, CAP_HWCAP, HWCAP_FP),
>       HWCAP_CAP(SYS_ID_AA64PFR0_EL1, ID_AA64PFR0_FP_SHIFT, FTR_SIGNED, 1, CAP_HWCAP, HWCAP_FPHP),
>       HWCAP_CAP(SYS_ID_AA64PFR0_EL1, ID_AA64PFR0_ASIMD_SHIFT, FTR_SIGNED, 0, CAP_HWCAP, HWCAP_ASIMD),
> diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
> index 1e25545..7f94623 100644
> --- a/arch/arm64/kernel/cpuinfo.c
> +++ b/arch/arm64/kernel/cpuinfo.c
> @@ -76,6 +76,7 @@
>       "asimddp",
>       "sha512",
>       "sve",
> +     "asimdfhm",
>       NULL
>  };
>
> --
> 1.9.1
>
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