Re: [PATCH 3/8] parisc: implement dma_mmap_coherent()

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

On Fri, Jul 10, 2009 at 03:11:29PM +0000, James Bottomley wrote:
> The design of coherent memory was for memory based device mailboxes
> managed by the kernel ... trying to give userspace coherent access to
> the same mailbox is problematic because it gives a direct way for the
> process to interfere with a device function ... shouldn't whatever
> you're trying to do be better accomplished by using an API to control
> the device and keeping the coherent mailbox fully in the kernel address
> space?

As far as sound DMA goes, it's not about mailboxes.  It's about a circular
buffer which you want the device to DMA from direct to/from the DAC/ADC
and have the application write/read data directly to/from that same
buffer.

Without this, you end up having to copy the sound data - at something
around 200KB/s from applications into a driver managed buffer, which is
quite an unnecessary overhead for the CPU.

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