Re: can device drivers return non-ram via vm_ops->nopage?

[Jeff Garzik <jgarzik@pobox.com>]

William Lee Irwin III wrote:
> int dma_mmap_coherent_sg(struct dma_scatterlist *sglist,
>                         int nr_sglist_elements,     /* length of sglist */
>                         struct vm_area_struct *vma, /* for address space */
>                         unsigned long address,      /* user virtual address */
>                         unsigned long offset,       /* offset (in pages) */
>                         unsigned long nr_pages);    /* length (in pages) */
> 
> int dma_munmap_coherent_sg(struct dma_scatterlist *sglist,
>                         int nr_sglist_elements,     /* length of sglist */
>                         struct vm_area_struct *vma, /* for address space */
>                         unsigned long address,      /* user virtual address */
>                         unsigned long offset,       /* offset (in pages) */
>                         unsigned long nr_pages);    /* length (in pages) */
> 
> int dma_alloc_coherent_sg(struct dma_scatterlist **sglist,
>                         unsigned long length);      /* length in pages */
> 
> int dma_free_coherent_sg(struct dma_scatterlist **sglist,
>                         unsigned long length);      /* length in pages */

No comment on struct dma_scatterlist, but the above is the most natural 
API for audio drivers at least.

Audio drivers allocate buffers at ->probe() or open(2), and the only 
entity that actually cares about the contents of the buffers are (a) the 
hardware and (b) userland.  via82cxxx_audio only uses 
pci_alloc_consistent because there's not a more appropriate DMA 
allocator for the use to which that memory is put.

Audio drivers only need to read/write the buffers inside the kernel when 
implementing read(2) and write(2) via copy_{to,from}_user().

	Jeff