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: -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
