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Subject: RE: Reserving a fixed physical address page of RAM.
Date: Tue, 28 Nov 2006 14:37:32 -0500
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Thread-Topic: Reserving a fixed physical address page of RAM.
Thread-Index: AccS1gpnhJbSJlOTRQmYG98ggWbD8wATSASQ
From: "Jon Ringle" <JRingle@vertical.com>
To: "Jesper Juhl" <jesper.juhl@gmail.com>,
       "Fawad Lateef" <fawadlateef@gmail.com>
Cc: "Dave Airlie" <airlied@gmail.com>, "Robert Hancock" <hancockr@shaw.ca>,
       <linux-kernel@vger.kernel.org>
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Jesper Juhl wrote:
> On 28/11/06, Fawad Lateef <fawadlateef@gmail.com> wrote:
> > On 11/28/06, Dave Airlie <airlied@gmail.com> wrote:
> > > On 11/28/06, Jon Ringle <jringle@vertical.com> wrote:
> > <snip>
> > > > It looks promising, however, I need to reserve a 
> physical address 
> > > > area that is well known (so that the code running on the other 
> > > > processor knows where in PCI memory to write to). It 
> appears that 
> > > > dma_alloc_coherent returns the address that it 
> allocated. Instead 
> > > > I need something where I can tell it what physical 
> address and range I want to use.
> > > >
> > >
> > > I've seen other projects just boot a 128M board with mem=120M and 
> > > just use the 8MB at 120 to talk to the other processor..
> > >
> >
> > Yes, this can be used if required physical-memory exists in 
> the last 
> > part of RAM as if you use mem=<xxxM> then kernel will only 
> use memory 
> > less than or equal-to <xxxM> and above can be used by 
> drivers (or any 
> > kernel module) might be through ioremap which takes 
> physical-address.
> >
> > But if lets say we need only 1MB portion of specific 
> physical-memory 
> > region then AFAIK it must be done by hacking in kernel code during 
> > memory-initialization (mem_init function) where it is 
> marking/checking 
> > pages as/are reserved; you can simply mark you required pages as 
> > reserved too and set their count to some-value if you want to know 
> > later which pages are reserved by you. (can do this reservation work
> > here: http://lxr.free-electrons.com/source/arch/i386/mm/init.c#605).
> > CMIIW
> >
> 
> Can you not use the 'memmap=' kernel option to reserve the 
> specific area you need?
> (See Documentation/kernel-parameters.txt for details)

This looks like what I want, but after searching for this, I found that
memmap= is implemented for i386 and ia64 only. My arch is arm (processor
is an IXP455).

I'll explore the mem= option to see if it will work out.

Jon
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