audio: drop --softvol=no and --softvol=auto

Drop the code for switching the volume options and properties between
af_volume and AO volume controls. interface-changes.rst mentions the
changes in detail.

Do this because this was exceedingly complex and had other problems as
well. It was also very hard to test. It's just not worth the trouble.

Some leftovers like AOCONTROL_HAS_PER_APP_VOLUME will be removed at a
later point.

Fixes #3322.

3 files changed, 51 insertions, 37 deletions

diff --git a/DOCS/interface-changes.rst b/DOCS/interface-changes.rst
index 99f426be14..d2e1ee4e7f 100644
--- a/DOCS/interface-changes.rst
+++ b/DOCS/interface-changes.rst
@@ -21,6 +21,23 @@ Interface changes
 
  --- mpv 0.18.1 ---
     - deprecate --heartbeat-cmd
+    - remove --softvol=no capability:
+        - deprecate --softvol, it now does nothing
+        - --volume, --mute, and the corrsponding properties now always control
+          softvol, and behave as expected without surprises (e.g. you can set
+          them normally while no audio is initialized)
+        - rename --softvol-max to --volume-max (deprecated alias is added)
+        - the --volume-restore-data option and property are removed without
+          replacement. They were _always_ internal, and used for watch-later
+          resume/restore. Now --volume/--mute are saved directly instead.
+        - the previous point means resuming files with older watch-later configs
+          will print an error about missing --volume-restore-data (which you can
+          ignore), and will not restore the previous value
+        - as a consequence, volume controls will no longer control PulseAudio
+          per-application value, or use the system mixer's per-application
+          volume processing
+        - system or per-application volume can still be controlled with the
+          ao-volume and ao-mute properties (there are no command line options)
  --- mpv 0.18.0 ---
     - now ab-loops are active even if one of the "ab-loop-a"/"-b" properties is
       unset ("no"), in which case the start of the file is used if the A loop
diff --git a/DOCS/man/input.rst b/DOCS/man/input.rst
index 74f7815d40..1d8bfe30cd 100644
--- a/DOCS/man/input.rst
+++ b/DOCS/man/input.rst
@@ -1234,29 +1234,31 @@ Property list
     See ``--hr-seek``.
 
 ``mixer-active``
-    Return ``yes`` if the audio mixer is active, ``no`` otherwise. This has
-    implications for ``--softvol=no`` mode: if the mixer is inactive, changing
-    ``volume`` doesn't actually change anything on the system mixer. If the
-    ``--volume`` or ``--mute`` option are used, these might not be applied
-    properly until the mixer becomes active either. (The options, if set, will
-    just overwrite the mixer state at audio initialization.)
+    Return ``yes`` if the audio mixer is active, ``no`` otherwise.
 
-    While the behavior with ``mixer-active==yes`` is relatively well-defined,
-    the ``no`` case will provide possibly wrong or insignificant values.
-
-    Note that an active mixer does not necessarily imply active audio output,
-    although this is implied in the current implementation.
+    This option is relatively useless. Before mpv 0.18.1, it could be used to
+    infer behavior of the ``volume`` property.
 
 ``volume`` (RW)
-    Current volume (see ``--volume`` for details). Also see ``mixer-active``
-    property.
+    Current volume (see ``--volume`` for details).
 
-``volume-max``
-    Current maximum value the volume property can be set to. (This may depend
-    on the ``--softvol-max`` option.)
+``volume-max`` (RW)
+    Current maximum value the volume property can be set to. (Equivalent to the
+    ``--volume-max`` property.)
 
 ``mute`` (RW)
-    Current mute status (``yes``/``no``). Also see ``mixer-active`` property.
+    Current mute status (``yes``/``no``).
+
+``ao-volume`` (RW)
+    System volume. This property is available only if mpv audio output is
+    currently active, and only if the underlying implementation supports volume
+    control. What this option does depends on the API. For example, on ALSA
+    this usually changes system-wide audio, while with PulseAudio this controls
+    per-application volume.
+
+``ao-mute`` (RW)
+    Similar to ``ao-volume``, but controls the mute state. May be unimplemented
+    even if ``ao-volume`` works.
 
 ``audio-delay`` (RW)
     See ``--audio-delay``.
diff --git a/DOCS/man/options.rst b/DOCS/man/options.rst
index 1a5298f903..f9c32e5a9d 100644
--- a/DOCS/man/options.rst
+++ b/DOCS/man/options.rst
@@ -1072,14 +1072,10 @@ Audio
 
 ``--volume=<value>``
     Set the startup volume. 0 means silence, 100 means no volume reduction or
-    amplification. A value of -1 (the default) will not change the volume. See
-    also ``--softvol``.
+    amplification. Negative values can be passed for compatibility, but are
+    treated as 0.
 
-    .. note::
-
-        This was changed after the mpv 0.9 release. Before that, 100 actually
-        meant maximum volume. At the same time, the volume scale was made cubic,
-        so the old values won't match up with the new ones anyway.
+    Since mpv 0.18.1, this always controls the internal mixer (aka "softvol").
 
 ``--audio-delay=<sec>``
     Audio delay in seconds (positive or negative float value). Positive values
@@ -1094,20 +1090,17 @@ Audio
 
     See also: ``--volume``.
 
-``--softvol=<mode>``
-    Control whether to use the volume controls of the audio output driver or
-    the internal mpv volume filter.
+``--softvol=<no|yes|auto>``
+    Deprecated/unfunctional. Before mpv 0.18.1, this used to control whether
+    to use the volume controls of the audio output driver or the internal mpv
+    volume filter.
 
-    :no:    prefer audio driver controls, use the volume filter only if
-            absolutely needed
-    :yes:   always use the volume filter
-    :auto:  prefer the volume filter if the audio driver uses the system mixer
-            (default)
+    The current behavior is as if this option was set to ``yes``. The other
+    behaviors are not available anymore, although ``auto`` almost matches
+    current behavior in most cases.
 
-    The intention of ``auto`` is to avoid changing system mixer settings from
-    within mpv with default settings. mpv is a video player, not a mixer panel.
-    On the other hand, mixer controls are enabled for sound servers like
-    PulseAudio, which provide per-application volume.
+    The ``no`` behavior is still partially available through the ``ao-volume``
+    and ``ao-mute`` properties. But there are no options to reset these.
 
 ``--audio-demuxer=<[+]name>``
     Use this audio demuxer type when using ``--audio-file``. Use a '+' before
@@ -1264,10 +1257,12 @@ Audio
     their start timestamps differ, and then video timing is gradually adjusted
     if necessary to reach correct synchronization later.
 
-``--softvol-max=<100.0-1000.0>``
+``--volume-max=<100.0-1000.0>``, ``--softvol-max=<...>``
     Set the maximum amplification level in percent (default: 130). A value of
     130 will allow you to adjust the volume up to about double the normal level.
 
+    ``--softvol-max`` is a deprecated alias and should not be used.
+
 ``--audio-file-auto=<no|exact|fuzzy|all>``, ``--no-audio-file-auto``
     Load additional audio files matching the video filename. The parameter
     specifies how external audio files are matched. ``exact`` is enabled by