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

[Jens Axboe <axboe@suse.de>]

On Thu, Mar 02 2006, linux-os (Dick Johnson) wrote:
> 
> 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

A highmem page is a regular page, it just doesn't have a kernel virtual
mapping. You can dma to/from these pages just fine.

> 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.

Nonsense. You can't have multiple iommu mappings or dma transactions
against the same highmem page of course, however that is no different
than a low mem page.

-- 
Jens Axboe