linux-kernel - Re: [PATCH] powerpc/mm: setting mmaped page cache property through device tree

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Message-Id: <E389CC83-307F-4A7D-B90B-CD928A1D1D1D@kernel.crashing.org>
Date:	Tue, 1 Dec 2009 15:35:55 +0100
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Li Yang <leoli@...escale.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linuxppc-dev@...abs.org, paulus@...ba.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/mm: setting mmaped page cache property through  device tree

> 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,
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
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.

Segher

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