linux-kernel - Re: [PATCH v11 1/4] arm64: cpufeature: introduce helper cpu_has_hw

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Date:   Fri, 11 Oct 2019 11:38:58 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     "Justin He (Arm Technology China)" <Justin.He@....com>
Cc:     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-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "hejianet@...il.com" <hejianet@...il.com>,
        "Kaly Xin (Arm Technology China)" <Kaly.Xin@....com>,
        nd <nd@....com>
Subject: Re: [PATCH v11 1/4] arm64: cpufeature: introduce helper
 cpu_has_hw_af()

On Fri, Oct 11, 2019 at 01:16:36AM +0000, Justin He (Arm Technology China) wrote:
> From: Catalin Marinas <catalin.marinas@....com>
> > On Wed, Oct 09, 2019 at 04:42:43PM +0800, Jia He wrote:
> > > +		u64 mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);
> > > +
> > > +		return !!cpuid_feature_extract_unsigned_field(mmfr1,
> > > +
> > 	ID_AA64MMFR1_HADBS_SHIFT);
> > 
> > No need for !!, the return type is a bool already.
> 
> But cpuid_feature_extract_unsigned_field has the return type "unsigned int" [1]
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/include/asm/cpufeature.h#n444

And the C language gives you the automatic conversion from unsigned int
to bool without the need for !!. The reason we use !! in some places is
for converting long to int (not bool) and losing the top 32-bit. See
commit 84fe6826c28f ("arm64: mm: Add double logical invert to pte
accessors") for an explanation.

-- 
Catalin