[Bug 1882787] [NEW] AUD_set_volume_out takes SWVoiceOut as parameter, but controls HWVoiceOut

* [Bug 1882787] [NEW] AUD_set_volume_out takes SWVoiceOut as parameter,  but controls HWVoiceOut
@ 2020-06-09 14:18 malib69055
  2021-05-01  8:38 ` [Bug 1882787] " Thomas Huth
  0 siblings, 1 reply; 2+ messages in thread
From: malib69055 @ 2020-06-09 14:18 UTC (permalink / raw)
  To: qemu-devel

Public bug reported:

There was a change in
https://github.com/qemu/qemu/commit/571a8c522e0095239598347ac0add93337c1e0bf
#diff-230ab01fa7fb1668a1e9183241115cb0R1852-R1853 (audio/audio.c) which
breaks audio output on devices which have multiple software voices on
the same hardware voice.

When multiple software voices use the same hardware voice, then setting
a volume / mute for any of the software voices, will affect all other
software voices, too.

I'm not sure if such a use-case exists in QEMU; however, it does exist in my fork.
It's also easy to see that this is a bug in the QEMU audio subsystem.

The API (and broken function) for this is:

```
    void AUD_set_volume_out (SWVoiceOut *sw, int mute, uint8_t lvol, uint8_t rvol)
```

So this is supposed to modify the SWVoiceOut.

However, if the backend supports `pcm_ops->volume_out` this does not
work as expected. It's always as if you had called:

```
    void AUD_set_volume_out (HWVoiceOut *hw, int mute, uint8_t lvol, uint8_t rvol)
```

*(Note how this modifies the hardware voice)*

In my specific use case, I have 2 outputs (digital and analog audio on AC97), and I want to mute the digital audio output, but I still need to keep the voice activated for timing.
However, if I mute the digital audio SWVoiceOut, it will also affect the other SWVoiceOut (for analog audio) on the same HWVoiceOut.

---

Old code - if the hardware supports volume changes, it will receive the
software voice which should be modified, so changes can be restricted to
that one voice:

```
        HWVoiceOut *hw = sw->hw;
        [...]
        if (hw->pcm_ops->ctl_out) {
            hw->pcm_ops->ctl_out (hw, VOICE_VOLUME, sw);
        }
```

New code - the hardware backend will have no way to differentiate
software voices; so any change will affect all voices. The volume which
was set last on any sw voice will set a global volume / mute for all
voices on the hardware backend:

```
        HWVoiceOut *hw = sw->hw;
        [...]
        if (hw->pcm_ops->volume_out) {
            hw->pcm_ops->volume_out(hw, &sw->vol);
        }
```

The old interface was already broken with some (all?) backends, because they ignored the software voice, but at least the design made sense.
However, the new design is fundamentally broken because it doesn't even tell the backend which voice is supposed to be modified.

---

Bug was introduced in cecc1e79bf9ad9a0e2d3ce513d4f71792a0985f6
The affected code was touched since then, but still remains in 49ee11555262a256afec592dfed7c5902d5eefd2

** Affects: qemu
     Importance: Undecided
         Status: New

** Tags: audio subsystem voice

-- 
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https://bugs.launchpad.net/bugs/1882787

Title:
  AUD_set_volume_out takes SWVoiceOut as parameter, but controls
  HWVoiceOut

Status in QEMU:
  New

Bug description:
  There was a change in
  https://github.com/qemu/qemu/commit/571a8c522e0095239598347ac0add93337c1e0bf
  #diff-230ab01fa7fb1668a1e9183241115cb0R1852-R1853 (audio/audio.c)
  which breaks audio output on devices which have multiple software
  voices on the same hardware voice.

  When multiple software voices use the same hardware voice, then
  setting a volume / mute for any of the software voices, will affect
  all other software voices, too.

  I'm not sure if such a use-case exists in QEMU; however, it does exist in my fork.
  It's also easy to see that this is a bug in the QEMU audio subsystem.

  The API (and broken function) for this is:

  ```
      void AUD_set_volume_out (SWVoiceOut *sw, int mute, uint8_t lvol, uint8_t rvol)
  ```

  So this is supposed to modify the SWVoiceOut.

  However, if the backend supports `pcm_ops->volume_out` this does not
  work as expected. It's always as if you had called:

  ```
      void AUD_set_volume_out (HWVoiceOut *hw, int mute, uint8_t lvol, uint8_t rvol)
  ```

  *(Note how this modifies the hardware voice)*

  In my specific use case, I have 2 outputs (digital and analog audio on AC97), and I want to mute the digital audio output, but I still need to keep the voice activated for timing.
  However, if I mute the digital audio SWVoiceOut, it will also affect the other SWVoiceOut (for analog audio) on the same HWVoiceOut.

  ---

  Old code - if the hardware supports volume changes, it will receive
  the software voice which should be modified, so changes can be
  restricted to that one voice:

  ```
          HWVoiceOut *hw = sw->hw;
          [...]
          if (hw->pcm_ops->ctl_out) {
              hw->pcm_ops->ctl_out (hw, VOICE_VOLUME, sw);
          }
  ```

  New code - the hardware backend will have no way to differentiate
  software voices; so any change will affect all voices. The volume
  which was set last on any sw voice will set a global volume / mute for
  all voices on the hardware backend:

  ```
          HWVoiceOut *hw = sw->hw;
          [...]
          if (hw->pcm_ops->volume_out) {
              hw->pcm_ops->volume_out(hw, &sw->vol);
          }
  ```

  The old interface was already broken with some (all?) backends, because they ignored the software voice, but at least the design made sense.
  However, the new design is fundamentally broken because it doesn't even tell the backend which voice is supposed to be modified.

  ---

  Bug was introduced in cecc1e79bf9ad9a0e2d3ce513d4f71792a0985f6
  The affected code was touched since then, but still remains in 49ee11555262a256afec592dfed7c5902d5eefd2

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