linux-kernel - Re: lsl / lsr possible confusion in v7_flush_dcache

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Message-ID: <20160908093801.GB1493@arm.com>
Date:   Thu, 8 Sep 2016 10:38:01 +0100
From:   Will Deacon <will.deacon@....com>
To:     Vincent Siles <vincent.siles@...venrun.com>
Cc:     Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: lsl / lsr possible confusion in v7_flush_dcache_all

On Thu, Sep 08, 2016 at 11:15:20AM +0200, Vincent Siles wrote:
> While reading the v7_flush_dcache_all (arch/arm/mm/cache-v7.S), I
> stumbled upon this line:
> 
> # r10 is the current cache level
> 127: add    r2, r10, r10, lsr #1	    @ work out 3x current cache level
> 
> If we want r2 to be 3 * r10, we should compute r10 + (r10 << 1), which
> is lsl, not lsr.
> 
> I check for a recent kernel, the issue seems to still be here:
> repo:       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> revision:   d71f058617564750261b673ea9b3352382b9cde4

This code is take pretty much verbatim from the ARM ARM and, despite
being fairly obfuscated, does what it says on the tin. r10 is incremented
by 2 each time round the loop, so this is basically doing 2i + (2i / 2).

Will