[openib-general] assumptions on page mapping (was High memory)

[Christoph Hellwig]

On Thu, Dec 08, 2005 at 09:17:15AM +0200, Or Gerlitz wrote:
> Roland Dreier wrote:
> >The right way to use the MR from get_dma_mr() is to use "bus
> >addresses" from the DMA mapping API.  For highmem, the right way to
> >get those addresses is with dma_map_sg() or dma_map_page().
> 
> Looking on the kernel x86_64 code, both dma_map_sg and dma_map_page seem 
> to assume that the page is already mapped, since they call 
> page_address(page).

x86_64 doesn't have highmem, so page_address(page) is valid on every
page.

> Specifically is it safe in a SCSI LLD (eg SRP and iSER which is among 
> other things such) to call dma_map_sg on a SG which comes with a SCSI 

yes, this is definitly safe.  

> command, so the SCSI Mid-Layer always makes sure the pages are mapped?

no, it doesn't.  in fact pages don't need to be mapped at all for dma
normally.