> I need an API to transfer audio data from userspace to kernelspace. I > initially thought about ALSA, but it turns out some assumptions made > by ALSA are not fulfilled by my system. One of the most serious > problems is that the UAC gadget driver doesn't have any audio clock. > The only hardware clock available is the USB device controller > interrupts generated at the USB transfer rate, and those are much > faster than the audio sample rate. This will cause buffer underruns > that I need to handle. You don't need a clock. The data will come to you at the right rate. All you need to do is pass it on when you have enough to fill up a buffer. The buffer size is fixed. It's part of the spec for the device you are emulating. Assume that you get samples one at a time each time the source clock ticked. The normal state of your system would be to have a buffer that is partially filled. When a new sample fills up the buffer, you would move it from the input side to the ready-for-USB queue and setup a new buffer for future input samples. Soon the USB side will read the queued buffer, you free that buffer and you are now back to the normal state of collecting input data. You are in trouble if that doesn't happen before the nest buffer is ready, that is the ready-for-USB queue should normally be empty. If you can't keep it empty it will eventually overflow. Short chunks of time where the queue builds up might be OK. For debugging, you should probably count them. Now consider the case where you get several samples at a time rather than one. Logically, copy them over one at a time. That turns into a burst of source clocks, but the average will work out right. Your ready-for-USB queue should still be empty most of the time. If that doesn't make sense, I'll try again. Consider something like audio over a network. There is no audio clock on the network. The receiver can derive the source clock by watching the data stream. -- These are my opinions, not necessarily my employer's. I hate spam. -- 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
