linux-kernel - Re: [PATCH] ARM: ptrace: Use bitfield helpers

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Message-ID: <YZvCtN/WNyogs9JZ@shell.armlinux.org.uk>
Date:   Mon, 22 Nov 2021 16:17:56 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: ptrace: Use bitfield helpers

On Mon, Nov 22, 2021 at 04:53:41PM +0100, Geert Uytterhoeven wrote:
> The isa_mode() macro extracts two fields, and recombines them into a
> single value.  The shift value of the J-bit may look off-by-one to the
> casual reader, as it is the net result of the extraction and
> recombination steps.

I'd recommend avoiding the suggestion that the explicit "- 1" could
be misinterpreted as an off-by-one error.

>  #define isa_mode(regs) \
> -	((((regs)->ARM_cpsr & PSR_J_BIT) >> (__ffs(PSR_J_BIT) - 1)) | \
> -	 (((regs)->ARM_cpsr & PSR_T_BIT) >> (__ffs(PSR_T_BIT))))
> +	((FIELD_GET(PSR_J_BIT, (regs)->ARM_cpsr) << 1) | \

I'd suggest getting rid of the extra unnecessary parens there.
Too many parens leads to a decrease in readability.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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