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Date:   Tue, 18 May 2021 18:26:54 -0400 (EDT)
From:   Nicolas Pitre <nico@fluxnic.net>
To:     Linus Walleij <linus.walleij@linaro.org>
cc:     "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>,
        Yanfei Xu <yanfei.xu@windriver.com>,
        Mike Rapoport <rppt@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hailong liu <carver4lio@163.com>,
        Tian Tao <tiantao6@hisilicon.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/4] ARM: change vmalloc_min to vmalloc_start
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References: <20210518120633.GW12395@shell.armlinux.org.uk> <E1liyda-0005B4-Kk@rmk-PC.armlinux.org.uk> <CACRpkdbZObW2SXdTUkPrsezjjVU19emts420EN-uhkHWb+4vrA@mail.gmail.com>
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On Wed, 19 May 2021, Linus Walleij wrote:

> On Tue, May 18, 2021 at 2:15 PM Russell King (Oracle)
> <rmk+kernel@armlinux.org.uk> wrote:
> 
> > Change the current vmalloc_min, which is supposed to be the lowest
> > address of vmalloc space including the VMALLOC_OFFSET, to vmalloc_start
> > which does not include VMALLOC_OFFSET.
> >
> > Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> 
> Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
> 
> > +static unsigned long __initdata vmalloc_start = VMALLOC_END - (240 << 20);
> 
> When I first read this it took me some time to figure out what was
> going on here, so if you have time, please fold in a comment
> with some explanation of that (240 << 20) thing, in some blog
> post I described it as "an interesting way to write 0x0f000000"
> but I suppose commit 0536bdf33faf chose this way for a
> specific reason? (Paging Nico if he can explain it.)

That's an alternative (and deprecated) way to write MB(240).


Nicolas