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

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Sat, Mar 20, 2004 at 02:45:18PM -0800, William Lee Irwin III wrote:
> Is there any possibility of an extension to remap_area_pages() that
> could resolve this? I can't say I fully understood and/or remember
> the issue with it that you pointed out.

The issues are:

1. ALSA wants to mmap the buffer used to transfer data to/from the
   card into user space.  This buffer may be direct-mapped RAM,
   memory allocated via dma_alloc_coherent(), an on-device buffer,
   or anything else.

   The user space mapping must likewise be DMA-coherent.

   Currently, ALSA just does virt_to_page() on whatever address it
   feels like in its nopage() function, which is obviously not
   acceptable for two out of the three specific cases above.

2. ALSA wants to _coherently_ share data between the kernel-side
   drivers, and user space ALSA library, mainly the DMA buffer
   head/tail pointers so both kernel space and user space knows
   when the buffer is full/empty.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 PCMCIA      - http://pcmcia.arm.linux.org.uk/
                 2.6 Serial core