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    SubjectRe: [PATCH] powerpc/mm: setting mmaped page cache property through device tree
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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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