[alsa-devel] [PATCH V1 09/11] ASoC: DaVinci: i2s, add davinci_i2s_prepare and shutdown

[Ambrose, Martin]

On Sat, Jul 04, 2009 at 21:29:59, Troy Kisky wrote:

> If the codec is master then prepare should call
> mcbsp_start, not trigger.

For me this change induces audio artifacts when starting or stopping a gstreamer pipeline.
Take, e.g., the pipeline
	gst-launch -v filesrc location=file.mp3 ! mad ! alsasink

In the setup, before the file is opened, gstreamer places the pipeline into the PAUSE state.
	... posting state-changed READY to PAUSED

This results in a call to the davinci_i2s_prepare function which, when the codec is master,
stops then starts the mcbsp driver. However at this point in the pipeline no valid data has
been provided to the DMA layer (davinci-pcm). Consequently there is random noise of somewhat
random length played out the DAC at the start of file playback. Moreover when the stream changes
from the PAUSE to the PLAYING state another mcbsp_stop/mcbsp_start sequence is performed which
seems inefficient. Similar behavior, and noise, occurs when the pipeline is shutdown.

The aplay utility has a simpler startup/shutdown procedure which doesn't produce any noise.

There doesn't seem to be any state context, e.g. pause vs start, provided to the prepare
function so I can't see how to handle such cases in the current driver while preserving
the use of this patch. If I revert this patch then I don't get any noise but I don't 
know what the consequences are. 

Comments/suggestions welcome.

Regards,
Martin