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

From: gengdongjiu
Date: Mon Dec 11 2017 - 07:47:43 EST


On 2017/12/11 19:59, Dave P Martin wrote:
> 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.Thanks for the point out.
In fact, this feature only adds two instructions:
FP16 * FP16 + FP32
FP16 * FP16 - FP32

The spec call this bit to ID_AA64ISAR0_EL1.FHM, I do not know why it will call
"FHM", I think call it "FMLXL" may be better, which can stand for FMLAL/FMLSL instructions.

>
> 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 ;)
how about something like "performing a multiplication of each FP16 element of one
vector with the corresponding FP16 element of a second vector, and to
add or subtract this without an intermediate rounding to the
corresponding FP32 element in a third vector."?

>
>> instructions set. Let the userspace know about it via a
>> HWCAP bit and MRS emulation.
>>
>> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>> Signed-off-by: Dongjiu Geng <gengdongjiu@xxxxxxxxxx>
>> ---
>> 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.
I will update it, thanks for the point out

>
> Otherwise, looks OK.
Appreciate for your review.

>
> 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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