linux-kernel - Re: [PATCH 2/5] powerpc: Replace _ALIGN_DOWN() by ALIGN

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Date:   Tue, 21 Apr 2020 10:55:16 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Joel Stanley <joel@....id.au>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        linux-fbdev@...r.kernel.org, kvm@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel@...ts.freedesktop.org, Paul Mackerras <paulus@...ba.org>,
        alsa-devel@...a-project.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/5] powerpc: Replace _ALIGN_DOWN() by ALIGN_DOWN()

Hi!

On Tue, Apr 21, 2020 at 01:04:05AM +0000, Joel Stanley wrote:
> On Mon, 20 Apr 2020 at 18:38, Christophe Leroy <christophe.leroy@....fr> wrote:
> > _ALIGN_DOWN() is specific to powerpc
> > ALIGN_DOWN() is generic and does the same
> >
> > Replace _ALIGN_DOWN() by ALIGN_DOWN()
> 
> This one is a bit less obvious. It becomes (leaving the typeof's alone
> for clarity):
> 
> -((addr)&(~((typeof(addr))(size)-1)))
> +((((addr) - ((size) - 1)) + ((typeof(addr))(size) - 1)) &
> ~((typeof(addr))(size)-1))
> 
> Which I assume the compiler will sort out?

[ This is line-wrapped, something in your mailer?  Took me a bit to figure
  out the - and + are diff -u things :-) ]

In the common case where size is a constant integer power of two, the
compiler will have no problem with this.  But why do so complicated?

Why are the casts there, btw?


Segher