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SubjectRe: [PATCH 2.6.22.y] ieee1394: revert "sbp2: enforce 32bit DMA mapping"
Stefan Richter wrote:
> Benjamin Herrenschmidt wrote:
>> Oh and, don't do the set_dma_mask() in sbp2, it has nothing to do there.
>> It should be in the ohci1394 driver.
>
> That's not quite right. OHCI-1394 implementations can go beyond 4GB bus
> address space. (Although I don't know if there are such implementations
> available. At least there are two implementations which can set the
> so-called Physical Range bigger than 4GB.)
>
> Sbp2 however requires that everything which it DMA-maps resides in the
> Physical Range of the controller. This way the CPU is not involved in
> most of the data transfers. The OHCI-1394 controller acts as bus bridge
> between IEEE 1394 bus and local bus, with a 1:1 mapping of IEEE 1394 bus
> addresses to and from local bus addresses --- but not in the whole 48
> bits white IEEE 1394 bus address range, only in the
> implementation-dependent Physical Range. The minimum Physical Range
> that all OHCI-1394 implementations guarantee is 4GB. I could actually
> have set a bigger mask in sbp2 when the controller supports a
> programmable bigger range.
>
> So that's the story why that dma_set_mask went into sbp2: Sbp2 wants
> mappings in a _subset_ of the OHCI-1394 controllers DMA range.
>
> Anyway. For now I will simply go with what 2.6.23-rc has and what
> 2.6.21 had: No dma_set_mask anywhere in the 1394 subsystem. We can
> revisit this whenever an actual need arises.

Not sure this is a very good idea. This seems rather likely to fail on
x86_64 machines with >4GB of RAM for example..

--
Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

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