Stefan Esser wrote:
Scott Long schrieb:
Jan Mikkelsen wrote:
Hi,
Doug White wrote:
On Fri, 22 Sep 2006, Jan Mikkelsen wrote:
Quick summary: sym(4) assumes on amd64 that virtual
addresses provided by
bus_dmamem_alloc() have the same alignment as the physical
addresses (in
this case, 2*PAGE_SIZE). They don't, and stuff breaks.
This patch works
around that.
Why is this? busdma supports alignment constraints; why not just set
the alignment to what you need it set at? I realize sym has its own
hand rolled DMA management craziness but alignment is something
busdma can take care of easily.
sym has the alignment requirement on the virtual address because of the
buddy memory allocation algorithm; changing how sym allocates memory
internally would remove the requirement. The buddy algorithm with 2^13
bytes aligned on a 2^12 byte (but not a 2^13 byte) boundary can
provide two
chunks of 2^12 bytes but nothing greater than 2^12 bytes.
The VTOBUS failure is caused by the buddy implementation making alignment
assumptions which aren't true, and then getting the virtual addresses
wrong.
Perhaps I'm just doing something wrong with bus_dma. I believe I set the
alignment requirements to be 2*PAGE_SIZE, and this is what I see for the
physical address. However the virtual address seems to only be page
aligned.
I can't see any mention of virtual address alignment in the bus_dma man
page. Can it take care of virtual address alignment? If so, how?
busdma makes no guarantees on virtual addresses.
Sigh, sorry I never got this fixed. The custom memory allocator made me
unhappy, and I never had time to dig into it. Do real docs on sym exist
somewhere? I'm not against sitting down and re-writing the physical
memory handling to both work and conform to the FreeBSD APIs.
I've been the co-author of the ncr SCSI driver, on which sym is based
(though not that particular code fragment). Since I know the structure
and principals of the driver (and since I have and know the docs up to
the 53c875, possibly also the 53c895), I'd probably be in a position
to work on this with the least effort to get started. Only problem is
that I do not have an amd64 system for testing ...
I changed the private allocator in the sym driver to use contigmalloc,
some time ago, but now I understand that there are stricter alignment
requirements. For a start, a work-around could be committed, IMHO (even
if it is ugly). The better approach is of course an extension of busdma
to support aligned physical chunks as required by the driver.
But I could also try to find a clean fix for the affected driver code.
Is the Symbios SCSI controller still used that much that the effort
required for a "clean" fix is well spent?
Regards, STefan
What does the driver need that busdma doesn't provide now?
bus_dmamem_alloc() is just a wrapper around contigmalloc. All alignment
and exclusion arguments that you can give contigmalloc are honored by
busdma. I guess that's what frustrated me when I looked at this driver;
I couldn't figure out why all the effort had been made to have a custom
allocator based on contigmalloc (which should be an interface hidden
from most drivers and available only to the MD portion of the kernel,
btw) when busdma works fine for nearly every other driver out there.
I'm not being critical here, it's just that I couldn't figure out if
there was some subtle detail that I was missing. If there are
facilities that busdma doesn't provide but that you need, I'd be very
happy to work with you on developing them.
Scott
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