[alsa-devel] TWL4030 low-latency
peter.ujfalusi at ti.com
Tue Feb 14 08:46:15 CET 2012
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
> 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
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?
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