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Date: Wed, 30 Jan 2013 11:14:20 +0000
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: walter harms <wharms@bfs.de>
Cc: Joe Perches <joe@perches.com>, Julia Lawall <julia.lawall@lip6.fr>,
        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
Subject: Re: coccinelle and bitmask arithmetic
Message-ID: <20130130111420.GG23505@n2100.arm.linux.org.uk>
References: <20130127194039.GA18787@elgon.mountain> <1359317078.14406.12.camel@joe-AO722> <20130127201947.GO16282@mwanda> <9561.1359474916@turing-police.cc.vt.edu> <1359475998.4196.26.camel@joe-AO722> <alpine.DEB.2.02.1301291847070.3339@hadrien> <1359482637.15135.7.camel@joe-AO722> <5108D808.6010405@bfs.de>
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On Wed, Jan 30, 2013 at 09:21:28AM +0100, walter harms wrote:
> Great hit Joe :)
> 
> Sometimes i am really surprised what code can be found
> in the kernal and it is still working.
> Having no clue of the code i suspect somebody tries to
> check is mask outside the range it should read
> PHYS_OFFSET |( SZ_64M - 1)
> maybe someone should tell them that
> 1+1=10 while 1|1=1
> It does not seem to matter here (or ... ?)

This PCI host is only used on one platform (ARMCORE).

For this, PHYS_OFFSET will be a value with only the top few bits of a
32-bit word set (such as 0xc0000000) - it's certainly not going to have
any bits set below bit 26 on the platform this driver gets used on.
"SZ_64M - 1" is the size of the window that RAM appears.

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.