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 04:06:21PM +0100, Andrea Arcangeli wrote:
> On Sat, Mar 20, 2004 at 06:40:22AM -0800, William Lee Irwin III wrote:
> > On Sat, Mar 20, 2004 at 02:30:25PM +0100, Andrea Arcangeli wrote:
> > > Anyways returning to the non-ram returned by ->nopage see the below
> > > email exchange with Jens. the bug triggering of course is the
> > > BUG_ON(!pfn_valid(page_to_pfn(new_page))).
> > > If we want to return non-ram, we could, but I believe we should change
> > > the API to return a pfn not a page_t * if we want to.
> > 
> > This would be very helpful for other reasons also. There's a general
> > API issue with drivers that want or need to do this. The one I've
> 
> I'm afraid I'll have to teach ->nopage how to deal with non-ram with
> this page_t API too (changing it to pfn sounds too intrusive in the
> short term), it seems to me that alsa can return non-ram (in the nopage
> callback there's a virt_to_page on some iomm region), and changing alsa
> to use remap_file_pages sounds too intrusive too. 

Actually, ALSA is broken in that respect - it isn't portable as it
stands.  It isn't the API which is broken - it's ALSA which is broken.
Performing virt_to_page() on any non-direct mapped RAM page (which
means the value returned from dma_alloc_coherent or pci_alloc_consistent)
is undefined.

One of my current projects is fixing this crap in ALSA.

Besides, returning an invalid struct page will lead to Bad Things(tm)
in set_pte().

-- 
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