linux-kernel - RE: Reserving a fixed physical address page of RAM.

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Message-ID: <22170ADB26112F478A4E293FF9D449F44D0F7D@secure.comdial.com>
Date:	Tue, 28 Nov 2006 14:26:03 -0500
From:	"Jon Ringle" <JRingle@...tical.com>
To:	"Robert Hancock" <hancockr@...w.ca>
Cc:	<linux-kernel@...r.kernel.org>
Subject: RE: Reserving a fixed physical address page of RAM.

Robert Hancock wrote:
> Jon Ringle wrote:
> > Robert Hancock wrote:
> >> Jon Ringle wrote:
> >>> Hi,
> >>>
> >>> I need to reserve a page of memory at a specific area of RAM that 
> >>> will be used as a "shared memory" with another processor 
> over PCI. 
> >>> How can I ensure that the this area of RAM gets reseved 
> so that the 
> >>> Linux's memory management (kmalloc() and friends) don't use it?
> >>>
> >>> Some things that I've considered are iotable_init() and ioremap().
> >>> However, I've seen these used for memory mapped IO 
> devices which are 
> >>> outside of the RAM memory. Can I use them for reseving RAM too?
> >>>
> >>> I appreciate any advice in this regard.
> >>
> >> Sounds to me like dma_alloc_coherent is what you want..
> >>
> > 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 don't think this is possible in the general case, as 
> there's no mechanism for moving things out of the way if they 
> might be in use. Your best solution is likely to use 
> dma_alloc_coherent and pass the bus address returned into the 
> other processor to tell it where to write..

I can't do that because my mechanism to talk to the other processor is
exactly what I'm trying to setup. Catch-22 :)
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