At Wed, 24 Jul 2013 10:41:43 -0400 (EDT), Alan Stern wrote: > > I have been studying the data_ep_set_params() function in > sound/usb/endpoint.c. This is the routine that calculates the number > of samples and I/O requests to keep on the USB hardware queue for PCM > audio, based on the ALSA parameters. > > It uses the PERIOD_BYTES parameter but not BUFFER_BYTES. In simplified > terms (ignoring rounding, boundary cases, and other things), the number > of periods per buffer is fixed at 24 for recording and 1 for playback, > completely ignoring the user's setting. If you look at the parameters > copied in snd_usb_hw_params() in sound/usb/pcm.c, you'll see what I > mean. > > Is this really the intended behavior? It doesn't seem right at all. The buffer size doesn't matter for urb setup because the usb-audio driver transfers the data by the driver itself at urb completes. The buffer size is considered in these callbacks, i.e. prepare_playback_urb() and retire_capture_urb(). OTOH, the period size is evaluated for determining the urb buffer size, so that the data transfer (thus the wakeup via complete) is aligned to the period size. Takashi -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
