On 08/19/2009 03:44 AM, Ira W. Snyder wrote:
You don't need in fact a third mode. You can mmap the x86 address space
into your ppc userspace and use the second mode. All you need then is
the dma engine glue and byte swapping.
Hmm, I'll have to think about that.
The ppc is a 32-bit processor, so it has 4GB of address space for
everything, including PCI, SDRAM, flash memory, and all other
peripherals.
This is exactly like 32bit x86, where you cannot have a PCI card that
exposes a 4GB PCI BAR. The system would have no address space left for
its own SDRAM.
(you actually can, since x86 has a 36-40 bit physical address space even
with a 32-bit virtual address space, but that doesn't help you).
On my x86 computers, I only have 1GB of physical RAM, and so the ppc's
have plenty of room in their address spaces to map the entire x86 RAM
into their own address space. That is exactly what I do now. Accesses to
ppc physical address 0x80000000 "magically" hit x86 physical address
0x0.
So if you mmap() that, you could work with virtual addresses. It may be
more efficient to work with physical addresses directly though.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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