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| Is it possible to use Speakup across multiple users on Linux? This article is the notes and ramblings of someone who tries to do that. | | Is it possible to play audio 'globally' or system-wide in Linux? This article is the notes and ramblings of someone who tries to do that. |
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| == Background == | | == Background == |
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| Speakup alone is not enough to implement a screen reader: It still needs some speech synthesizer to turn text and control signals in to audio. Once upon a time hardware synthesizers connected over serial were used to do this task, but as computers became more capable software synthesizers became viable. | | Speakup alone is not enough to implement a screen reader: It still needs some speech synthesizer to turn text and control signals in to audio. Once upon a time hardware synthesizers connected over serial were used to do this task, but as computers became more capable software synthesizers became viable. |
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| Speakup supports software synthesizers by exposing a character device in /dev/softsynthu. A connector such as connects to this device and implements the synthesizer protocol and handles playback of speech. In practice the software synthesizer is [https://github.com/linux-speakup/espeakup espeakup] which plays speech using [https://github.com/espeak-ng/espeak-ng eSpeak NG]. | | Speakup supports software synthesizers by exposing a character device in /dev/softsynth. A connector such as connects to this device and implements the synthesizer protocol and handles playback of speech. In practice the software synthesizer is [https://github.com/linux-speakup/espeakup espeakup] which plays speech using [https://github.com/espeak-ng/espeak-ng eSpeak NG]. |
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| == The problem == | | == The problem == |
| Linux is a multi-user system, but hardware and other resources are not. There's only one sound card and only one Speakup device. So these resources have to be shared somehow. | | Linux is a multi-user system and |
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| Since around 2007 with the release of [https://www.freedesktop.org/wiki/Software/ConsoleKit/ ConsoleKit] the Linux desktop has handled sharing resources using the idea of seats. To put it simply:
| | - speakup is system-wide, not multi-user. so it must always have access to the audio device. the problem is that this isn't possible in linux. |
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| * Devices get allocated to seats
| | - espeakup runs as root boot up and log in as root- works fine |
| * Logging in gives you a session
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| * Only one session can use a seat at a time
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| * Switching sessions requires handing over devices to another session
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| The Speakup device has no support for sharing, but the sound card does.
| | - log in as yourself- no audio, pulseaudio claims the sound card. or tries to and gives up because root is playing audio, leaving you without sound |
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| This leads to the following situation:
| | - run espeakup as your own user- you can read what root does, but switching to a root tty now goes silent |
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| # Linux boots
| | - run pulseaudio systemwide is not recommended, you can snoop data |
| # systemd gives root the current audio device
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| # systemd starts espeakup as root
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| # You can read the login prompt or login as root
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| # You log in as your own user
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| # systemd gives your user the current audio device
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| # Your PulseAudio instance claims the device
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| # The root espeakup can no longer speak
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| # You can no longer use Speakup
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| # You might have no sound at all if PulseAudio couldn't claim the device if espeakup was still talking while PulseAudio was starting
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| # You switch to another TTY using Ctrl+Alt+F2
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| # systemd gives root the current audio device
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| # Your PulseAudio instance frees the device
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| # espeakup can talk again using the sound card
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| Because espeakup can only speak when root is using the current seat effectively becomes useless outside logging in as root on your computer.
| | - pipewire |
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| It's also important to note that PulseAudio stores settings for volume and outputs. You might be using Bluetooth headphones at low volume but then switch to a root TTY and have espeakup blare loudly out your desktop speakers.
| | - pulseaudio |
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| It really makes me wonder why PulseAudio doesn't have a per-seat instance that lets you switch between sessions and preserves audio configuration but still swaps out which user's applications can use audio.
| | == Linux audio architecture == |
| | learn what's actually going on here |
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| As a quick note: PipeWire has the exact same behaviour, so for the most part you can substitute 'PulseAudio' with 'PipeWire' or any sound other server when reading this page.
| | hardware: |
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| == Existing solutions ==
| | - oss |
| Here are some solutions that kind of work but have severe trade-offs.
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| === Running PulseAudio system-wide ===
| | - alsa |
| Running system-wide PulseAudio allows you to always have espeakup running and talking regardless of who is currently logged in.
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| However this has a major security concern, mainly that all users can now play and record audio from your sound card and other applications. For more information see [https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/WhatIsWrongWithSystemWide/ What is wrong with system mode?]
| | - audio group |
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| Running PipeWire system-wide is a little more complicated: You need to run both the PipeWire daemon and a session manager system-wide. This session manager needs to lock hardware and not give it up on seat switch.
| | sound servers: |
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| I'm not going to include instructions on how to do this as I can't provide support for it. Sorry.
| | - dmix |
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| === Running espeakup as your user ===
| | - esd |
| It's possible to run espeakup as your own user by:
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| * Taking control of the audio device away from logind
| | - arts |
| * Giving control of all audio devices to your user
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| * Giving control of the softsynth to your user
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| * Running espeakup as a daemon as your user
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| * Running espeakup at boot as your user
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| I've prepared steps to follow to get this working below.
| | - phonon |
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| Paste lines between <code>--- PASTE START ---</code> and <code>--- PASTE END ---</code> in to the file specified or a terminal.
| | - pulseaudio |
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| Step 1: Put this in <code>/etc/modules-load.d/speakup.conf</code>
| | - auth using a cookie (even dmix) |
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| --- PASTE START ---
| | - include multiple |
| speakup
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| speakup_soft
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| --- PASTE END ---
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| This will cause the speakup kernel modules to load at boot.
| | one per user? |
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| Step 2: Put this in <code>/etc/udev/rules.d/99-speakup.rules</code>
| | - handover during login |
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| --- PASTE START ---
| | - handover during seat swapping |
| SUBSYSTEM=="sound", TAG-="seat", GROUP="audio"
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| KERNEL=="softsynth*", GROUP="audio"
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| --- PASTE END ---
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| This will will do three things:
| | - there's no system-wide audio |
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| # Stop logind managing sound devices
| | TODO: when did sound servers lock hardware? |
| # Give users in the audio group access to sound devices
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| # Give users in the audio group access to Speakup
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| Step 3: Put this in <code>~/.config/systemd/user/espeakup.service</code>
| | TODO: when did sound servers give up hardware for seats? |
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| --- PASTE START ---
| | system and pulseaudio coordinate will snatch the audio whenever you change to a seat assigned to your user. maybe you could modify these to not cede the audio outside a specific seat (in this case seat = virtual tty you switch in linux with ctrl alt f1 through f7)? |
| [Unit]
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| Description=Software speech output for Speakup
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| After=pulseaudio.service
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| [Service]
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| Environment="default_voice= ALSA_CARD="
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| ExecStart=/usr/bin/espeakup -d --default-voice=${default_voice}
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| Restart=always
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| Nice=-10
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| OOMScoreAdjust=-900
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| [Install]
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| WantedBy=default.target
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| --- PASTE END ---
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| This is a service that just runs the espeakup daemon. It is set to start after pulseaudio.
| | here's some more nightmare stuff i realized: seats only give permission to device nodes. applications still have to gracefully hand off the hardware during a seat switch. pulse does this just by shutting up |
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| Change <code>pulseaudio.service</code> to <code>pipewire.service</code> if you're using on PipeWire.
| | pulseaudio doesn't gracefully give up audio, only alsa -> pulseaudio |
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| Step 4: Run these commands in a terminal as your user:
| | - pulseaudio startup race |
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| --- PASTE START ---
| | - pipewire has most of the same problems |
| systemctl --user enable espeakup
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| loginctl enable-linger
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| sudo gpasswd -a $USER audio
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| --- PASTE END ---
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| This does three things:
| | == Attempt 1: Sharing Speakup == |
| | - have a proxy that sits between speakup and espeak |
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| # Enable the speakup user service
| | - send messages to espeakup instances based on current active UID |
| # Enable running user services when logged out
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| # Add your user to the audio group
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| Step 5: Reboot and enjoy!
| | - during a switch between instances, wait for the current instance to finish talking OR the stop talking control is sent. then start feeding the new instance data |
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| There's two downsides to this method:
| | - have a shim that blocks pulseaudio from starting until it has permission, but also don't consume the buffer |
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| # Your user can see what other users are reading, including root
| | i do not like how i'm basically reinventing flow control but poorly |
| # Other users can't play audio
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| == Ideas ==
| | ok so it turns out i was WRONG: you can't share speakup protocol between multiple synths! the protocol is stateful! ie if you tell it to change voice and switch synth the voice change won't be applied. YAY |
| There are some ideas I've considered to solve this problem.
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| === Loaning PulseAudio to root === | | == Attempt 2: Sharing synth data == |
| The main case here would be to allow root to use a user's PulseAudio install.
| | so this makes the only viable solution to send PCM data from a root espeakup instance. |
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| This would have mean that multiple people can privately use a computer, with the exception of one user being able to read root's screen.
| | on top of that this also means i have to modify espeakup to handle some flow control AND output to a buffer instead of the sound card |
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| I'm not too sure what this buys compared to using sudo or something to act as root.
| | == Attempt 3: One Pulse for all == |
| | | - user pulse for all system |
| In practice I'm not sure how easy this would be to implement. You would need to replace the logind hardware management for sound with something else that allows finer grained management of hardware.
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| === Sharing Speakup between multiple users === | |
| Things would be a lot easier if we could run one espeakup instance per user. This is tricky because the Speakup kernel modules don't have concepts of users or sessions.
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| There's a wide list of engineering problems to solve with this:
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| * Saving and restoring per-user Speakup settings
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| * Saving and restoring the softsynth state between users
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| * Proxying /dev/softsynth so users can't read other's data
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| * Restricting access to the fakekey input device
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| * Handling graceful handovers from ALSA to PulseAudio
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| * Handling forceful handovers from PulseAudio to ALSA
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| * Shim PulseAudio so it waits for hardware access before running
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| * Flow control to indicate when the proxy is ready to send
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| * Flow control to indicate when espeakup is ready to talk
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| * Buffering data when espeakup isn't ready
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| * Discarding buffered data when speakup signals espeakup to shut up
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| * Handling messages sent back from espeakup to speakup and flow control for that
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| There's a lot to untangle here since we're touching multiple levels of abstraction.
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| === Sharing espeakup output between multiple users ===
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| A more practical solution might be to run a root espeakup instance and proxy its PCM output to users.
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| This would cut down the engineering problems to:
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| * Assigning /sys/accessibility/speakup to the current seat
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| * Proxying PCM data to stub programs users run
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| * Handling graceful handovers from ALSA to PulseAudio
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| * Shim PulseAudio so it waits for hardware access before running
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| * Flow control to indicate when a proxy is ready to send
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| * Buffering data when a stub isn't playing audio
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| * Discarding buffered data when espeakup wants to shut up
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| This gives most the benefits of sharing speakup but without saving or restoring state.
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| [[Category:Research]]
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