Hi Jeffrey,

sorry for the late reply.

On 08.11.2012 19:54, Jeffrey Barish wrote:

On Wed 07 November 2012 10:01:14 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.

Here's the diff. I can't figure out how to submit a patch.

--- sound/usb/pcm.c.orig 2012-11-08 11:43:57.309732860 -0700 +++ sound/usb/pcm.c 2012-11-08 11:41:06.512420658 -0700 @@ -456,7 +456,7 @@ static int configure_endpoint(struct snd return ret;

    if (subs->sync_endpoint)

•           ret = snd_usb_endpoint_set_params(subs->data_endpoint,
  

•           ret = snd_usb_endpoint_set_params(subs->sync_endpoint,
                                              subs->pcm_format,
                                              subs->channels,
                                              subs->period_bytes,
  

Thanks - I just sent out a patch for this.

Daniel