linux-kernel - Re: [PATCH v4 1/3] arm64: cpufeature: introduce helper cpu_has_hw

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Date:   Wed, 18 Sep 2019 15:20:41 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Jia He <justin.he@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Marc Zyngier <maz@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     Punit Agrawal <punitagrawal@...il.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Jun Yao <yaojun8558363@...il.com>,
        Alex Van Brunt <avanbrunt@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>, hejianet@...il.com,
        Kaly Xin <Kaly.Xin@....com>
Subject: Re: [PATCH v4 1/3] arm64: cpufeature: introduce helper
 cpu_has_hw_af()

Hi Jia,

On 18/09/2019 14:19, Jia He wrote:
> We unconditionally set the HW_AFDBM capability and only enable it on
> CPUs which really have the feature. But sometimes we need to know
> whether this cpu has the capability of HW AF. So decouple AF from
> DBM by new helper cpu_has_hw_af().
> 
> Signed-off-by: Jia He <justin.he@....com>
> Suggested-by: Suzuki Poulose <Suzuki.Poulose@....com>
> ---
>   arch/arm64/include/asm/cpufeature.h | 1 +
>   arch/arm64/kernel/cpufeature.c      | 6 ++++++
>   2 files changed, 7 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index c96ffa4722d3..206b6e3954cf 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -390,6 +390,7 @@ extern DECLARE_BITMAP(boot_capabilities, ARM64_NPATCHABLE);
>   	for_each_set_bit(cap, cpu_hwcaps, ARM64_NCAPS)
>   
>   bool this_cpu_has_cap(unsigned int cap);
> +bool cpu_has_hw_af(void);
>   void cpu_set_feature(unsigned int num);
>   bool cpu_have_feature(unsigned int num);
>   unsigned long cpu_get_elf_hwcap(void);
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index b1fdc486aed8..c5097f58649d 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1141,6 +1141,12 @@ static bool has_hw_dbm(const struct arm64_cpu_capabilities *cap,
>   	return true;
>   }
>   
> +/* Decouple AF from AFDBM. */
> +bool cpu_has_hw_af(void)
> +{
Sorry for not having asked this earlier. Are we interested in,

"whether *this* CPU has AF support ?" or "whether *at least one*
CPU has the AF support" ? The following code does the former.

> +	return (read_cpuid(ID_AA64MMFR1_EL1) & 0xf);

Getting the latter is tricky, and I think it is what we are looking
for here. In which case we may need something more to report this.

Kind regards
Suzuki