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Date: Wed, 2 Dec 2009 14:25:47 +0800
Message-ID: <2a27d3730912012225h3deab5dao2211bc404d247192@mail.gmail.com>
Subject: Re: [PATCH] powerpc/mm: setting mmaped page cache property through 
	device tree
From: Li Yang <leoli@freescale.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>, linuxppc-dev@ozlabs.org,
       paulus@samba.org, linux-kernel@vger.kernel.org
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On Tue, Dec 1, 2009 at 10:35 PM, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>> The scenario for the first case is that in a multicore system running
>> ASMP which means different OS runs on different cores.  They might
>> communicate through a shared memory region.  The region on every OS
>> need to be mapped with the same cache perperty to avoid cache paradox.
>
> This isn't true.  In ASMP, you cannot usually do coherency between
> the different CPUs at all.  Also, in most PowerPC implementations,

Coherency can't be achieved with proper configuration and management?  Why so?

> it is fine if one CPU maps a memory range as coherent while another
> maps it as non-coherent; sure, you have to be careful or you will

But we do want the shared region to be coherent.  So mappings should
have the same cacheability property.

> read stale data, but things won't wedge.
>
>> The scenario for the second case is to pre-allocate some memory to a
>> certain application or device (probably through mem=XXX kernel
>> parameter or limit through device tree).  The memory is not known to
>> kernel, but fully managed by the application/device.  We need being
>> able to map the region cachable for better performance.
>
> So make the memory known to the kernel, just tell the kernel not to
> use it.  If it's normal system RAM, just put it in the "memory" node
> and do a memreserve on it (or do something in your platform code); if
> it's some other memory, do a device driver for it, map it there.

Your solution is feasible.  But the memory allocation is a software
configuration.  IMHO, it should be better and easier addressed by
changing configurations(like mem parameter) rather than the kernel
platform code which should address hardware configuration.

- Leo
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