Adding Alsa Loopback Interface before build

olafzen · September 11, 2026 at 5:41 PM

Hello,

I tried to compile LE 12.2.1 with the option to use an Alsa loopback Interface.

I changed the Kernel config file (CONFIG_SND_ALOOP=m) first before the make image process.

But the modul has not been build, when compiling finished.

How is it possible, to build LE with the apropriate aloop modul? Either fixed in kernel or as a modul?

FYI; I want to use camilladsp with my LE Box (PI4b) and therefore I need a loopback interface.

Thanks in advance.

Best regards

Olaf

olafzen · September 12, 2026 at 8:24 AM

Nevermind,

I've read the GIT Wiki and will try that way (cloning, forking, editing and commiting...).

Is this the right way:

In my forked Git clone; edit /projects/RPi/devices/RPi4/linux/linux.aarch64.conf (add CONFIG_SND_ALOOP=m)

Add the changed file to the forked repo:

git add projects/RPi/devices/RPi4/linux/linux.aarch64.conf

Download sources:

PROJECT=RPi ARCH=aarch64 DEVICE=RPi4 tools/download-tool

Start build process:

PROJECT=RPi ARCH=aarch64 DEVICE=RPi4 make image

Test the new image. Check if the loopback module exists and loads (modprobe snd-aloop)

If succesfull:

git commit -m "linux: enable Alsa Loopback Interface"

???

**chewitt** · September 12, 2026 at 10:13 AM

If you have a working default image building, edit projects/RPi/devices/RPi4/linux/linux.aarch64.conf to enable the module then rebuild the image and it will rebuild the kernel and update the image.

At least that's the theory. It doesn't work for me on an Amlogic image with linux-7.3-rc2 kernel either, and diff comparing the edited conf and .config file from the kernel build folder shows the kernel has unset CONFIG_SND_ALOOP=m when processing the config; hence nothing gets built.

I will task Claude to investigate the problem once I take a break from some other debugging. I suspect some kind of upstream kernel issue and not something on our side.

**HiassofT** · September 12, 2026 at 10:34 AM

You also need CONFIG_SND_DRIVERS=y

so long,

Hias

**chewitt** · September 12, 2026 at 10:56 AM

Code

config SND_ALOOP                                                                          
        tristate "Generic loopback driver (PCM)"                                          
        select SND_PCM                                                                    
        select SND_TIMER                                                                  
        help                                                                              
          Say 'Y' or 'M' to include support for the PCM loopback device.                  
          This module returns played samples back to the user space using                 
          the standard ALSA PCM device. The devices are routed 0->1 and                   
          1->0, where first number is the playback PCM device and second                  
          number is the capture device. Module creates two PCM devices and                
          configured number of substreams (see the pcm_substreams module                  
          parameter).                                                                     
                                                                                          
          The loopback device allows time synchronization with an external                
          timing source using the time shift universal control (+-20%                     
          of system time).                                                                
                                                                                          
          To compile this driver as a module, choose M here: the module                   
          will be called snd-aloop.

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No mention of SND_DRIVERS there ^ so looks like a kernel bug to report and submit a fix for

**HiassofT** · September 12, 2026 at 11:02 AM

Scroll up a bit and you'll notice it's inside a if SND_DRIVERS block

linux/sound/drivers/Kconfig at master · torvalds/linux

Linux kernel source tree. Contribute to torvalds/linux development by creating an account on GitHub.

github.com

so long,

Hias

**chewitt** · September 12, 2026 at 11:05 AM

"User error: please replace user"

olafzen · September 12, 2026 at 11:37 AM

Ah OK, thanks for replies.

I guess the missing driver entry was the culprit in my 1st. attempt

Will give it a try.