On 01/12/2012 02:09 AM, Jacinto Alvarez wrote:

Hi Peter,

Thanks for your reply.

Not sure what you mean under latency. For me the latency is the time
needed to play out the sample at the DMA pointer in main memory.
In case of McBSP2 (which has 1280 word long buffer) it is maximum of:
(1280/<number of channels>)/sampling frequency
In case of stereo sample it is 14.51ms with 44.1KHz, and 13.3ms with
48KHz.

With latency I refer to the round-trip of the combined capture-playback operation. The application I am using creates a couple of input/output streams and bounds them together with snd_pcm_link(). The shortest time I can get for this round-trip is 1024 frames (~23 ms) and I get that by initializing the streams with 8 periods of 128 frames each.

In McBSP element mode the McBSP FIFO will be kept full all the time
(threshold is 0). This means that the McBSP will request one word from
the DMA if there's a single free slot in the FIFO.

If you switch to threshold mode things will be different under the hood
(providing better power saving):
the McBSP threshold value will be calculated according to the period
size. In this mode the DMA will request for a chunk of data based on the
threshold value, so the FIFO will be filled with DMA bursts. You still
have the FIFO caused latency, but the system can rest between bursts.
I think if you use McBSP threshold mode with 3x 5ms periods you should
be fine (the ALSA buffer is going to be 15ms).

Is this threshold mode set by the application with the snd_pcm_sw_params_set_start_threshold() command (e.g., passing a non-zero parameter) or are you talking of some tweak INSIDE the driver's code?

It is McBSP driver specific. we do not have (yet) ALSA level control for it. The best fit would be some type of QoS, but I'm not sure of it.

Currently the application is initializing the streams with snd_pcm_sw_params_set_start_threshold() and a value equal to the period size (128 frames).

Lets' try this (assuming you are using McBSP2): echo threshold > /sys/devices/platform/omap/omap-mcbsp.2/dma_op_mode

Do you see difference in the round-trip?