PCI DMA support on the MPC8250

Callebaut Benoit callebaut at mailandnews.com
Wed Jun 25 17:01:08 EST 2003

Hello Jim,
I am working on a MPC8245, and I had the same problem. There is no support in
Linux for CPU initiated DMA. The pci_consistent_alloc functions and all others
related to DMA like pci_sync_single have nothing to do with DMA. Their only
job is to keep a block of memory synchronized with the cache or disable the
use of cache with it.
But the DMA transfers are working and are 5 time faster. You must only program
them. It is just 5 registers to configure. You buffers in RAM must be
allocated with consistent_alloc to avoid caching on them and you must always
write physical addresses in the DMA registers( use virt_to_phys).
Note For  PCI addresses virt_to_phys doesn't work well. You must take the
addresses from the "remap" function code. I did it and it works very well.

A good advice : Try to reduce the setup code for the DMA to a maximum.
Otherwise you won't gain anything for small blocks (setup time > memcpy time

Or if you Firewire chip support DMA use the DMA ability of your Firewire chip.

Good luck

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