[alsa-devel] [Alsa-user] Ticks when playing to USB DAC at high sample rates

Daniel Mack zonque at gmail.com
Thu Nov 22 20:40:35 CET 2012

On 07.11.2012 18:05, Jeffrey Barish wrote:
> On Wed 07 November 2012 10:04:14 Daniel Mack wrote:
>> On 07.11.2012 10:01, Daniel Mack wrote:
>>> On 07.11.2012 00:54, Jeffrey Barish wrote:
>>>> I guess that snd_usb_pcm_prepare is called by the core usb system to
>>>> prepare the snd driver.  Anyway, snd_usb_pcm_prepare calls
>>>> configure_endpoint with a substream.  Then configure_endpoint calls
>>>> snd_usb_endpoint_set_params with the data endpoint that corresponds to
>>>> the substream.  snd_usb_endpoint_set_params calls data_ep_set_params
>>>> with the data ep and data_ep_set_params sets nurbs to 2 for the data
>>>> endpoint.
>>>> snd_usb_endpoint_set_params has code to call sync_ep_set_params when the
>>>> endpoint is type sync.  sync_ep_set_params sets nurbs to 4 (why?). 
>>>> However, snd_usb_endpoint_set_params never invokes the call to
>>>> sync_ep_set_params because ep->type is always data.  Back up:
>>>> snd_usb_endpoint_set_params is called from configure_endpoint.  It is
>>>> called from an if block contingent on the substream's having a sync
>>>> endpoint.  It does.  However, the call to snd_usb_endpoint_set_params
>>>> passes subs->data_endpoint as the first argument, so
>>>> snd_usb_endpoint_set_params simply repeats the operations it performed
>>>> as a result of the previous invocation.  I presume that the first
>>>> argument to the second call to snd_usb_endpoint_set_params should be
>>>> subs->sync_endpoint.> 
>>> Eh. You clearly spotted a bug here. The rally interesting thing is that
>>> that it didn't bite us earlier, as it's in since 3.6.
>>>>  I
>>>> made that change.  Now freqm does change.  However, I am getting tons of
>>>> ticks.  Now it sounds as if the payload size is always wrong.
>>>> Oh, 44.1 kHz works.  48 kHz works.  88.2 and 96 don't work.
>>> That's a different issue. Would you like to prepare a patch for the
>>> issue above please?
> I don't know how to prepare a patch, so I'm inclined to fix the remaining 
> problem before stopping to figure out the patch problem.
> I didn't get your previous email.  I hope that it contained some suggestions 
> on what to look for to solve the remaining problem.  Please send it again.  
> The problem seems to be related to timing considering that the driver works 
> perfectly 5-10% of the time, but timing of what?  I don't know what to look 
> for.

So with the first problem sorted out, I guess the other one is related
to to inexactly reported data feedback from the device. Some explanation
about the current code.

The code in the lower half in snd_usb_handle_sync_urb(), endpoint.c does
some auto-detection of the feedback rate format. This is done in order
to cope with all the many devices use have all sorts of rate formats.
When called for the first time after a stream start, it shifts the
received value until it matches the nominal sampling frequency with an
accuracy of +50% or -25%. Once the shift value is determined, it is
saved for later reference. In case the values get badly of of bounds
over subsequent calls, the code falls back to the auto-detection mode again.

In other words: if the device reports a bogus value in the very first
feedback packet, but stays inside the limits of the thresholds that
would make it fall back to auto-detection, the shift value will always
be wrong for the duration of that stream, resulting in bogus data rates
produced by the driver.

This is where I would have a look. Add some printk()s in the "if
(unlikely(ep->freqshift == INT_MIN))" case and dump the values that are
detected, for both a working and non-working case.

Let me know if that brings you anywhere :)


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