On Wed, Jun 8, 2011 at 12:13 AM, Mark Brown <broonie@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > On Tue, Jun 07, 2011 at 07:29:23PM +0100, Russell King - ARM Linux wrote: > >> 2. Circular buffer support has been added - see device_prep_dma_cyclic(). > >> However, 2 is not really a requirement for audio - you can queue several >> single slave transfers (one per period) initially, and then you get >> callbacks as each transfer completes. ÂIn the callback, you can submit >> an additional buffer, and continue doing so causing DMA to never end. > >> I believe that this is a saner approach than the circular buffer support, >> and its what I tried to put together for the AMBA PL041 AACI DMA (but >> unfortunately, ARMs platforms are totally broken when it comes to DMA.) > > Circular buffers are nice from the point of view of allowing you to > (providing the hardware supports it) totally disable the periodic audio > interrupts and leave the system to run for very long times off the > normal system timers. ÂThis gives a small but non-zero power benefit > providing the hardware gives you enough information about where the DMA > is so you can find out if you need to mix in a notification, otherwise > you get obvious latency issues. This is what I called free-running circular buffer. Besides power saving scenario, it is necessary for a fast peripheral with shallow fifo. The peripheral throws underrun errors, if the dma h/w doesn't support LLI and cpu takes a bit long loading-triggering the next transfer on DMA due to irq-latency for some reason. > You can also do this with an circular chain of sequential buffers of > course. This is what is called Circular buffer in Samsung's DMA API. -- To unsubscribe from this list: send the line "unsubscribe linux-samsung-soc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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