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

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

On Mon, Mar 22, 2004 at 12:58:07PM +0100, Andrea Arcangeli wrote:
> The only reason I believe a paging mechanism would been nicer, is that
> it would avoid latencies in dma_mmap_coherent (not necessairly scheduler
> latencies, but you would pay all the cost of the pagetables immediatly
> during the mmap syscall, so if you've to map gigs of ram that would tend
> to hang the task doing the mmap a little bit, I found it nicer to use
> the paging for this so we also only allocate the memory for the
> pagetables that we need, but OTOH Linus's right that in most cases it
> doesn't worth a single branch in a fast path).

However, if you go on to read what Linus said later, he seems to be saying
that we can guarantee that dma_alloc_coherent() will be backed by memory
which has page structures associated with it.  This means that we _can_
use the ->nopage function for the DMA coherent implementation after all.

However, it isn't useful for the PCI device-side buffer case, which would
need to be handled via remap_page_range().

-- 
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