Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?")

["David Xiao" <dxiao@broadcom.com>]

On Tue, 2009-08-25 at 05:53 -0700, Steven Walter wrote:
> On Thu, Aug 6, 2009 at 6:25 PM, Russell King - ARM
> Linux<linux@arm.linux.org.uk> wrote:
> [...]
> > As far as userspace DMA coherency, the only way you could do it with
> > current kernel APIs is by using get_user_pages(), creating a scatterlist
> > from those, and then passing it to dma_map_sg().  While the device has
> > ownership of the SG, userspace must _not_ touch the buffer until after
> > DMA has completed.
> [...]
> 
> Would that work on a processor with VIVT caches?  It seems not.  In
> particular, dma_map_page uses page_address to get a virtual address to
> pass to map_single().  map_single() in turn uses this address to
> perform cache maintenance.  Since page_address() returns the kernel
> virtual address, I don't see how any cache-lines for the userspace
> virtual address would get invalidated (for the DMA_FROM_DEVICE case).
> 
> If that's true, then what is the correct way to allow DMA to/from a
> userspace buffer with a VIVT cache?  If not true, what am I missing?

page_address() is basically returning page->virtual, which records the
virtual/physical mapping for both user/kernel space; and what only
matters there is highmem or not.

David