[alsa-devel] How does usbaudio synchronize on playback?
jcrowley at screenpc.com
Thu Nov 13 17:10:56 CET 2008
How is the playback data in usbaudio.c synchronized between the application
writing data to the runtime->dma_area buffer and the prepare_playback_urb
function copying this into URBs?
To start, a series of no-data URBs are sent out. The completion routine for
each URB then calls prepare_playback_urb, which copies more data into the
URB, and this URB is then passed to the USB system to be sent to the device.
prepare_playback_urb copies data from the runtime->dma_area to the
urb->transfer_buffer area. It maintains subs->hwptr_done to record what data
has been copied, and wraps at runtime->buffer_size since this is a circular
buffer. It also recognizes when runtime->period_size has been exceeded and
invokes snd_pcm_period_elapsed to tell the application that more playback
data is needed.
What I don't understand is how does prepare_playback_urb ensure that more
playback data has actually been written into the buffer?
In our case, we are streaming audio over a WAN, so we increased the number
of URBs to 16, with up to 32 packets per URB, to get a lot of data going
on-the-fly over a high latency link.
The problem is that we are getting erroneous data, and suspect that it is
caused by this synchronization issue. When the initial no-data URBs are
completed, the prepare_playback_urb logic copies a lot of data from the
buffer into new URBs -- but apparently no playback data has been written
into the buffer yet by the application. If we insert logic to arbitrarily
slow down the prepare_playback_urb logic, then the data appears to be
correct. But this is a (very) weak solution.
We also tried to save the runtime->control->appl_ptr value in order to
calculate how much new application data is available, but discovered that
appl_ptr is evidently not being updated (always zero). (running 2.6.23
Are we missing a key concept? Is there a way to reliably determine how much
data the application has actually written into the buffer?
Thanks for your help ...
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