thank you dave for your reply 

my processor is MPC8247, on Linux 2.6.11


in MPC8247 manual reference, the interrupt of DMA unit belonged PCI bridge.
that is different from SDMA and IDMA. through i do not know what 's the
different. heard SDMA is used to transfer between CPM and 60x. DMA is used
to transfer between 60x bus and PCI, or 60x bus - 60x bus. i am not sure.

Whether or not you get packing bytes when you access a
16-bit device and transfer the data to a 32-bit destination
depends on how the device is mapped. For example, a 16-bit
device can be implemented such that it responds to 8-bit,
16-bit, and 32-bit requests, but the 32-bit requests will
require more wait-states, since the device has to be
read from twice before constructing a 32-bit word to
place on the PCI bus.

ya, that's it

Its also possible that the DMA controller can be configured
to deal with different source and destination widths. However
without knowing what processor or DMA controller you are
asking about, theres not much to say.

in the manual reference charpter 9.13 DMA,  source and destination address
can be configured. i can not find the hint about source and destination
widths configured.
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