linux-kernel - Re: [PATCH] clocksource: mips-gic-timer: fix clocksource counter width

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Date:   Wed, 28 Feb 2018 10:19:25 +0100
From:   Felix Fietkau <nbd@....name>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org,
        paul.burton@...tec.com
Subject: Re: [PATCH] clocksource: mips-gic-timer: fix clocksource counter
 width

On 2018-02-28 09:56, Thomas Gleixner wrote:
> On Wed, 21 Feb 2018, Felix Fietkau wrote:
> 
>> This code needs to use ffs instead of fls on the mask to determine the
>> shift for reading the GIC_CONFIG_COUNTBITS field.
> 
> Why?
>>  	count_width = read_gic_config() & GIC_CONFIG_COUNTBITS;
>> -	count_width >>= __fls(GIC_CONFIG_COUNTBITS);
>> +	count_width >>= __ffs(GIC_CONFIG_COUNTBITS);
This code is trying to extract the GIC_CONFIG_COUNTBITS field from
read_gic_config(), so it needs to shift count_width right by the index
of the least significant bit (__ffs) instead of the most significant bit
(__fls) of the field mask.

- Felix