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

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

On Fri, Jul 10, 2009 at 06:30:50PM +0000, James Bottomley wrote:
> On Fri, 2009-07-10 at 19:16 +0100, Russell King wrote:
> > 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.
> 
> But that makes it sound like ordinary streaming DMA from a device to
> user space ... that's what the dma_map_xx APIs are already designed to
> handle:  I don't understand why you need coherent memory for this (which
> can be a scarce resource on some platforms).

The streaming APIs are inefficient for this.  Consider the overhead of
having to writeback and invalidate caches at 200KB/s (which is what
you're requiring ARM to do).  That's far too much CPU overhead.

It's much more efficient to use non-cached memory for this on ARM.

We've been doing this for years and years, it's well proven.

> Why?  The dma_map_xx API is designed to be zero copy.

Except with a rather large overhead of repetitive cache handling.

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