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Alexander Böttcher avatar

Native audio with Sculpt VC

On the way back of FOSDEM 2019 I recognized that I missed to setup audio for my Sculpt VC machine. I used it already with Sculpt TC, but somehow I never took the time to re-integrate it with Sculpt VC. The next days after the FOSDEM trip I consulted my personal audio advisor -Josef- how to setup things.

I document in the following my steps and some improvements in the hope they are useful to others. The steps are done on a machine running Sculpt VC natively, so it may not work when running Sculpt in a VM.

Genode has a port of a audio driver based on BSD drivers and a self written audio mixer. Additionally, a audio player using libav without a GUI exists in the Genode world repository. That is all nice, but do I have to build and package all myself ? Luckily - no!

My colleague Josef and others are more music affine then me and provide ready to use packages for Sculpt VC. So, lets go. Actually, the missing pieces are launcher configurations for the audio driver, audio mixer and the audio player.

For the audio driver I came up (with the help of Josef) with the following launcher snippet.


<launcher pkg="cnuke/pkg/bsd_audio_drv/2019-01-13">
       <config recording="false">
               <mixer field="outputs.master" value="128"/>
               <service name="Platform"> <parent label="audio"/> </service>
               <service name="RM">       <parent/> </service>

I decided to add the mixer already in beginning, so that several audio clients can be added later easily.


<launcher pkg="cnuke/pkg/mixer/2019-01-13">
               <default out_volume="100" volume="75"/>
               <service name="Audio_out"> <child name="audio_drv"/> </service>
               <service name="Report"> <parent/> </service>

Now we need a audio player, which connects to the mixer. For that I used the following launcher snippet.


<launcher pkg="cnuke/pkg/audio_player/2019-01-14">
       <!-- paused, playing, stopped -->
       <config state="paused">
               <report progress="yes" interval="10" playlist="yes"/>
               <service name="Audio_out"> <child name="mixer"/> </service>
               <service name="File_system"> <child name="audio_fs"/> </service>
               <service name="ROM" label="playlist"> <parent label="config -> playlist"/> </service>
               <service name="Report"> <parent/> </service>

As you may notice, the audio player needs access to a file system where the actual music files are located. For that I use a chroot component which gives readonly access to a subpath called audio.


 <launcher pkg="chroot">
               <service name="File_system"> <child name="default_fs_rw"/> </service>
       <config> <default-policy path="/audio" writeable="no"/> </config>

Additionally, one need to provide a playlist ROM, which contains the files to be played.


<playlist mode="once"> <!-- repeat -->
    <track path="/rock.mp3"/>
    <track path="/pop.mp3"/>
    <track path="/metal.mp3"/>

As you see, for the example I put in the audio directory 3 music files.

Now everything is in place. As soon when I changed the "paused" config option to "playing" things started to run. First I did not hear anything. After playing with the volume level in the mixer and audio driver launcher configs, I noticed that I had to unmute via the hardware keys Fn-F1 on a Lenovo T460. On a Lenovo X201 and T420 it just worked immediately. Thanks Josef !

The launcher config snippet are also available directly via my audio branch, if you build your Sculpt image yourself. Read the general information about building the boot image of Sculpt VC if you intend to do so.

At this point I could be done and enjoy music playing form a native Genode audio player on Genode's Sculpt VC.

The experience

Unfortunately, the sound sometimes stutter - especially when I put the machine under load, like running some load in Noux or in the Firefox@Seoul browser. Also switching into the Leitzentrale via F12 causes audible interruptions, which tend to happen several times to me. The main reason is that the audio components are running as part of the runtime subsystem which are prioritized below the Leitzentrale and the Drivers subsystems. (See system overview of Sculpt for details to the subsystems.

Sculpt VC is running on the NOVA kernel, which uses a pre-emptive fixed priority round robin scheduler. That means for our audio use-case, each component with a higher priority can interrupt as long and as often the audio components so that they miss all deadlines to deliver in-time audio data. One solution of course could be, to put all 3 audio components into the drivers subsystem.

This is somehow undesired, since it would increase the boot image size and actually audio is not strictly required to boot up a system, right ? Furthermore, the audio components would become running on a very high priority level which may consume close to all CPU time in the event of a bug.

Beside that, the general issue remains and you will encounter the same issue again whenever you have some kind of soft-realtime requirements, or more buzzword style, a mixed criticality system with untrusted components, e.g. ported drivers.

Feeling uncomfortable with the situation - I decided to look for a more general solution instead of a quick and dirty workaround (by moving it just to the drivers subsystem).

The foundations

For Genode's -hw- kernel we introduced long time ago a way to configure and to trade CPU time very similar to the RAM resource of a Genode component. There is also a FOSDEM presentation and video about the kernel and scheduler available and extensions with the 15.05 release. In short, you may configure CPU time for -hw- like:

<start name="audio_drv">
 <resource name="CPU" quantum="25"/>
 <resource name="RAM" quantum="4M"/>

Effectively, in the example the component gets guaranteed up to 25% of the CPU time. (For more details about the trading, periods etc. please read the link.) For our use-case it means, that the audio driver gets 25% assured time independent of any fixed priorities of others components. More importantly, if the audio driver goes nuts it can not run/spin/consume more than the 25%. Nice - that is what should improve life ! Of course the driver may get more than the 25% - if and only if there is slack CPU time left - which means that not all components consumed all their available CPU time.

The improvement

Inspired by the general concept the question is, can we also support it with the NOVA kernel ? The other direction - of just using -hw- as kernel for Sculpt VC - is not possible currently. Mainly, because of missing features on x86. However, a glimpse at our last roadmaps reveals that we are heading also into this direction.

Coming back to the NOVA kernel - the nice thing about this kernel is that it actually already supports since ages the possibility to attach to a thread several scheduling configurations (called scheduling context - SC).

The idea is now, that beside the normal SC we could attach a second SC. The normal SC has a priority and time quantum. Whenever the time quantum is depleted, it gets refilled immediately and the SC is attached at the end of the runnable list of SC of the given priority level.

The second SC, let it call periodic SC, requires additionally a time period. Within the given period, the periodic SC can run up to the specified time quantum. If the quantum of the SC gets depleted, it will be refilled -not- before the start of the next period. With this extra period configuration, we can express the mentioned 25% CPU time of the example - e.g. a quantum of 5ms in a period of 20ms. When the quantum of the periodic SC got depleted, the component still can continue to run on the normal SC, which is however subject to the normal fixed priority scheme.

Within one day a first experimental change to the NOVA kernel and Genode's core was usable. After some more days of testing it actually looked good. In the kernel the delayed re-filling of a quantum for a periodic SC has been added. In core the evaluation of the "CPU" resource parameter and the setup of the periodic SC have been added.

The testing

And now - does it actually help to mitigate our stutter issue for audio ?

First we have to add to our audio components a CPU quantum:

-<launcher pkg="cnuke/pkg/bsd_audio_drv/2019-01-13">
+<launcher cpu="10" pkg="cnuke/pkg/bsd_audio_drv/2019-01-13">
   <config recording="false">
     <mixer field="outputs.master" value="128"/>
-<launcher pkg="cnuke/pkg/audio_player/2019-01-14">
+<launcher cpu="10" pkg="cnuke/pkg/audio_player/2019-01-14">
   <!-- paused, playing, stopped -->
   <config state="paused">
     <report progress="yes" interval="10" playlist="yes"/>
-<launcher pkg="cnuke/pkg/mixer/2019-01-13">
+<launcher cpu="10" pkg="cnuke/pkg/mixer/2019-01-13">
     <default out_volume="100" volume="75"/>

After a rebuild of Sculpt VC and a reboot, it became not better - damn - still stuttering during pressing of F12.

After some while I noticed, that the CPU value is actually not evaluated by our sculpt_manager. The sculpt_manager actually transform the launcher code into the normal init <start ...> syntax. Ok, that is no big deal, just implement it by a small patch which is part of my audio branch.

Again. Sculpt VC image re-build. Copying image to USB.

Unplug USB from dev machine. Plug USB to test machine. Power on test machine.

Waiting ...

Clicking at "+" to start audio_drv, then the mixer and then audio_player.

Inspect the ram filesystem.

Editing with vim the config/audio_player file, change "paused" to "playing". Store it :w

The feeling


Pressing F12

Still sound - .oO - no stutter

Pressing F12 F12

Still sound without stutter - nice !!!

So - it works in principal. Still, there will be sound artifacts, but now it should happen much less. There are several unsolved minor and maybe major issues left, so please consider this branch and the kernel changes as highly experimental. I tested it only on few machines, namely on Lenovo X201, T420 and T460 - but I'm quite happy about the current state and the principal success.

I would be happy about some people testing it and give some feedback - especially your on-target experience would be of value for me.

For interested Genode developers: I adjusted the scenario in the Genode's world repository a bit to be really really evil to the audio subsystem, e.g. using CPU burners. You may use this simpler scenario then Sculpt for experiments.