bus_dmamem_alloc confusion

Chuck Tuffli chuck_tuffli at agilent.com
Fri Oct 15 21:00:10 PDT 2004

More weirdness in bus_dma land. The driver I'm working on (**** not in
the tree, but available if interested ****) allocates bus memory
according to a user space request and passes back a physical address.
The app in user space then does a mmap on the address to get a virtual
address that it can use. This has worked on both NetBSD and FreeBSD
but the user space mmap is failing on DFly.

The driver is using bus_dmamem_alloc() to allocate the memory and
bus_dmamap_load() to commit the region. The kva that comes back seems
fine (ie the driver can use it to write the memory), but I'm wondering
if the bus address is correct. For a 4096 byte allocation the kva and
bus address (b) were (a is the alignment)

Mem:  k=0xcc68f000 b=0x3c50000 a=0x1000

and for an 8096 byte allocation, the addresses come back

Mem:  k=0xcc6b4000 b=0x14000 a=0x2000

The user space app can't mmap either bus address. I could be way
wrong, but the second bus address looks suspicious to me.  This second
allocation goes through a different path in bus_dmamem_alloc() and
calls contigmalloc() instead of malloc() as would happen if the
request size <= PAGE_SIZE.

I steped through some of contigmalloc and saw the code checking the
request against bus addresses much lower than the 0x14000 (e.g.
0x2000) which makes me wonder about the bus addresses. Any suggestions
of what might be going on or what I should be looking at more closely?

---chuck (a newbie to the vm code)

Chuck Tuffli
Agilent Technologies

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