linux-kernel - Re: [PATCH v2] /dev/mem: use phys_addr

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Message-ID: <505112A3.7080703@ti.com>
Date:	Wed, 12 Sep 2012 18:54:27 -0400
From:	Cyril Chemparathy <cyril@...com>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	<linux-arm-kernel@...ts.infradead.org>,
	<linux-ia64@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-sh@...r.kernel.org>, <arnd@...db.de>, <dhowells@...hat.com>,
	<fenghua.yu@...el.com>, <lethal@...ux-sh.org>,
	<linux@....linux.org.uk>, <nico@...aro.org>,
	<rob.herring@...xeda.com>, <tony.luck@...el.com>,
	<tony@...mide.com>, <will.deacon@....com>
Subject: Re: [PATCH v2] /dev/mem: use phys_addr_t for physical addresses

Greg,

On 9/12/2012 2:15 PM, Greg KH wrote:
> On Wed, Sep 12, 2012 at 02:05:58PM -0400, Cyril Chemparathy wrote:
>> This patch fixes the /dev/mem driver to use phys_addr_t for physical
>> addresses.  This is required on PAE systems, especially those that run
>> entirely out of >4G physical memory space.
>
> What is the symptom if this patch is not applied?  Does it need to be
> backported to older kernels?  If so, which ones?
>

As far as I can tell, this change only benefits platforms where lowmem 
lies outside the 32-bit addressable range.  On the ARM based KeyStone 
platform, all of our memory is located above the 4GB limit at 
0x8:0000:0000.  On this platform, trying to read lowmem breaks as follows:

# dd if==/dev/mem of=/dev/null bs==4k count=1 skip=8388608
dd: /dev/mem: Bad address

Assuming that we are unique in this regard (i.e. lowmem outside 4GB), I 
don't see any reason to backport this to earlier kernels.

-- 
Thanks
- Cyril
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