22.06.2015 16:54, Raymond Yau wrote:
Are all soc audio driver use cyclic dma ?
I'm not sure I understand the question. All audio drivers use some kind of cyclic DMA.
The ALSA API requires the driver to provide a cyclic sample buffer (or something that behaves like one).
However, not all hardware works this way. USB and FireWire require the driver to continually queue new packets, whose size and timing are determined by the bus clock and are not directly related to the ALSA ring buffer. These drivers use double buffering; the actual DMA happens from those packets, not from the ring buffer.
If those queued packets/urb cannot be rewind, snd_pcm_rewindable should return zero for those driver
Not really.
As I understand it, the kernel periodically converts a piece of the ring buffer (located in RAM) into an URB, and it gets sent through the USB bus. Parts of the buffer that are not yet converted to URB are perfectly rewindable.
In other words, for USB devices, the kernel already implements the "low-latency background thread that makes unrewindable devices rewindable" idea that I discussed (as a strawman proposal) here for userspace:
http://mailman.alsa-project.org/pipermail/alsa-devel/2014-September/080868.h...