Patch: sym(4) "VTOBUS FAILED" panics on amd64, amd64/89550
scottl at samsco.org
Thu Sep 21 21:38:41 PDT 2006
Jan Mikkelsen wrote:
> 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
>>>this case, 2*PAGE_SIZE). They don't, and stuff breaks.
>>This patch works
>>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
> 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.
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