Re: [Jack-Devel] JACK2 for Android newly released!

On Tuesday 28 January 2014 14:23:30 김정연 wrote:
> > support the "USB audio class" standard protocol nowadays. Support for
> > such devices is provided by a standard Linux kernel module which is
> > platform independent.
> 
> Yes. android officially support USB Audio by framework level.
> But, how to JACK access and use /dev/usb/... as audio device using jack
> driver?
> I have no background about that.
> Are there any bridge between usb~alsa?

Like already suggested by Patrick, you need the ALSA USB audio kernel module
(modprobe snd-usb-audio), which already comes with the official Linux kernel. 
The driver module file should be at a location like:

	/lib/modules/KERNEL_NAME_AND_VERSION/kernel/sound/usb/snd-usb-audio.ko

By the way, devices under /dev/usb are just shown if there is a compatible 
driver available & loaded on the system. Otherwise the USB device will never 
appear under /dev/usb at all.

Once you loaded the kernel module, check whether the USB audio device is 
visible in the ALSA system, i.e with

	aplay -l

if yes, you can use i.e. "aplay -D DEVICENAME /foo/some.wav" to output some 
sound file to the USB audio device.

If you do not have that kernel module and are not permitted to add it to the 
Android release, you might solve it in user space. But that would mean quite 
some work: Since around kernel 2.6.2x there is something called "USB file 
system" (usbfs). If that is enabled in your kernel configuration, then every 
USB device attached will automatically appear under /proc/bus/usb/, that is no 
matter whether there is a driver for it or not. usbfs allows to communicate 
with USB devices in user space, by simply conveniently opening their "USB 
endpoint" file under /proc/bus/usb/ and reading/writing to it by a normal 
application. However you might then need to write such an application on your 
own, by looking at what snd-usb-audio for example does or by reading the "USB 
audio class" protocol definition, since I am not aware of some 
daemon/application ready to be used for that purpose.

If you solved that, you have the next problem: the JACK server is usually 
started with exactly one audio device. JACK2 has a slave driver concept, 
however I can't remember the details about it right now. Maybe Stéphane or 
somebody can comment it. I guess you would need to restart the JACK server 
each time a USB audio device was attached or detached, supplying the USB 
device(s) as slave device(s) at command line. Maybe you could also control 
that at runtime by dealing with JACK2's driver API.

> > Are you running a hypervisor kernel like OKL4 on lowest level and
> > Android/Linux on top of it? Or is the Android/Linux kernel running
> 
> directly on
> 
> > the ARM CPU?
> 
> I have no idea about hypervisor or something.

Hypervisor kernels are often used on mobile devices on lowest level. They are 
acting as kind of virtual machine layer for the "actual" operating systems 
running on top of the hypervisor kernel. This is usually done for two 
purposes: a) it allows multiple OSs to run at the same time, i.e. one OS for 
controlling embedded communication components, and in parallel the actual end 
user advertised OS like Android or Symbian. Further it allows b) the same OS 
binaries to run on a variety of hardware without modifications, saving 
development costs.

So whether or not there is a hypervisor kernel running, could have a large 
impact on the question whether you are actually able to run audio apps with 
low latency at all. So you might talk to somebody responsible for kernel level 
issues there at Samsung to find out whether that is the case.

Best regards,
Christian Schoenebeck

1390930167.2369_0.ltw:2, <201401281730.00658.schoenebeck at crudebyte dot com>