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 _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
