Re: virt_to_page/pci_map_page vs. pci_map_single

[Jamie Wellnitz]

On Mon, Nov 03, 2003 at 12:48:42PM -0600, James Bottomley wrote:
> 
> 
>     Jamie> The Document/DMA-mapping.txt in 2.6.0-test9 says "To map a
>     Jamie> single region, you do:" and then shows pci_map_single.  Is
>     Jamie> DMA-mapping.txt in need of patching?
>     
>     Sounds like it needs an update.
>     
> Erm, I don't think so.  pci_map_single() covers a different use case
> from pci_map_page().
> 
> The thing pci_map_single() can do that pci_map_page() can't is cope with
> contiguous regions greater than PAGE_SIZE in length (which you get
> either from kmalloc() or __get_free_pages()).  This feature is used in
> the SCSI layer for instance.

Does "pci_map_page(virt_to_page(addr))" handle contiguous regions of
multiple pages?  It seems like the i386 implementation will (assuming
we're dealing with kmalloc'd memory).  Although the semantics of
map_single seem better to me than map_page, if I'm mapping a single
"region" of multiple pages.

> 
> There has been talk of deprecating dma_map_single() in favour of
> dma_map_sg() (i.e. make all transfers use scatter/gather and eliminate
> dma_map_single() in favour of a single sg entry table) but nothing has
> been done about it (at least as far as I know).
> 
> James

Thanks,
Jamie Wellnitz
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