Re: Question: how to map SCSI data DMA address to virtual address?

[linux-os (Dick Johnson)]

On Thu, 2 Mar 2006, Jens Axboe wrote:

> On Thu, Mar 02 2006, linux-os (Dick Johnson) wrote:
>>
>> On Thu, 2 Mar 2006, Ju, Seokmann wrote:
>>
>>> Hi,
>>>
>>> In the 'scsi_cmnd' structure, there are two entries holding address
>>> information for data to be transferred. One is 'request_buffer' and the
>>> other one is 'buffer'.
>>> In case of 'use_sg' is non-zero, those entries indicates the address of
>>> the scatter-gather table.
>>>
>>> Is there way to get virtual address (so that the data could be accessed
>>> by the driver) of the actual data in the case of 'use_sg' is non-zero?
>>>
>>> Any comments would be appreciated.
>>>
>>>
>>> Thank you,
>>>
>>> Seokmann
>>> -
>>
>> There is a macro for this purpose. However, for experiments, in
>> the kernel, you can add PAGE_OFFSET (0xC00000000) to get the virtual
>> address. The macro is __va(a), its inverse is __pa(a).
>>
>> Careful. These things can change.
>
> Bzzt no, this wont work if an iommu is involved. It also wont work if
> the page doesn't have a virtual address mapping (think highmem).
>
> --
> Jens Axboe
>

Are you going to get DMA-able pages out of high memory? The guy is
doing scatter-table DMA, i.e., linked DMA. I think you need to
build that table with pages from get_dma_page(). If you use
highmem, somebody can program the iommu right out from under
the DMA engine while a DMA is in progress because the CPU is
not involved and can be executing lots of code that can do lots
of bad things.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.15.4 on an i686 machine (5589.54 BogoMips).
Warning : 98.36% of all statistics are fiction, book release in April.
_


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