Memory allocation/sharing when DMA operations used..

[Markus Kode Kaarn]

Hey everybody,

For last few days i came whith idea of how Device-Drivers, should handle DMA
requests. If DMA channels access can be considered as a priviledge of only
PLM's, or even centralized DMA driver(which is prefered), through which will
all DMA requests will be handled. First, i think DD's, that receives DMA
request from user for the first time, should allocate/map memory for it selt 
by requesting it from physmem. Then share this region of memory whith
task(as far as i can think these will be device drivers) that requests DMA.
There would be read-only and the writeable memory shared, every of which
is decided to be given whereas task want to receive(read) data, or
send(write) it.
Here i can see a good decision on that only a DMA driver should allocate and
share memory regions whith tasks requesting the operations. Because allowing
user to supply a buffer is not far clever, cause user can die or give up its
pages back to physmem, and at time of receive-operation from DMA channel
memory region could be used by some other task.

This probably can apply not only to DMA operations. Some system part that
provide service(s) to many tasks may can come whith this approach.

At the moment i don't know much about the memory allocation/sharing in hurd-l4
and can't be more specific on this, or supply code.

Comments please.