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Subject: Re: coccinelle and bitmask arithmetic
From: Joe Perches <joe@perches.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Julia Lawall <julia.lawall@lip6.fr>, walter harms <wharms@bfs.de>,
        Mike Rapoport <mike@compulab.co.il>, Valdis.Kletnieks@vt.edu,
        cocci@systeme.lip6.fr, Dan Carpenter <dan.carpenter@oracle.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.cz>, Paul Fulghum <paulkf@microgate.com>,
        David Howells <dhowells@redhat.com>, linux-kernel@vger.kernel.org,
        kernel-janitors@vger.kernel.org
Date: Wed, 30 Jan 2013 08:53:29 -0800
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On Wed, 2013-01-30 at 11:35 +0000, Russell King - ARM Linux wrote:
> On Wed, Jan 30, 2013 at 12:21:21PM +0100, Julia Lawall wrote:
> > On Wed, 30 Jan 2013, Russell King - ARM Linux wrote:
> > > So, _either_ logical OR or addition works.
> > >
> > > If we _did_ end up with a PHYS_OFFSET with bits less than bit 26 set
> > > here, we'd have bigger problems - because the base of RAM in PCI space
> > > will not correspond with PHYS_OFFSET and all the DMA mapping stuff breaks.
> > 
> > The "problem" is that the computation is done inconsistently within the
> > same file.  Sometimes with + and sometimes with |.
> 
> And I say that is not a problem; if it _does_ become a problem, there are
> bigger problems that would also need solving, which given a multi-subarch
> kernel become a lot lot harder.
> 
> Sure, we can just change them to all be a bitwise OR (sorry, not logical)
> but that's really only half the story.

As far as I can tell, there'd be a lot of +'s to
change in arch/arm and only 2 uses of | in it8152.c

$ git grep -P "\(?\s*SZ_\d+[A-Z]\s*-\s*1\s*\)?\s*\|" arch/arm
arch/arm/common/it8152.c:                       *dev->dma_mask = (SZ_64M - 1) | PHYS_OFFSET;
arch/arm/common/it8152.c:               dev->coherent_dma_mask = (SZ_64M - 1) | PHYS_OFFSET;

$ git grep -P "\+\s*\(?\s*SZ_\d+[A-Z]\b\s*-\s*1\s*\)?" arch/arm | wc -l
460

I think consistently using + would make it simpler
for some possible future conversion.