linking: Add ALSA loopback policy scripts

This policy script is in charge of linking all nodes from all ALSA loopback
collections.

src/config/wireplumber.conf

@@ -719,6 +719,14 @@ wireplumber.components = [
     provides = hooks.linking.default.target.get-filter-from
     requires = [ metadata.filters ]
   }
+  {
+    name = linking/alsa-loopback/find-filter-target.lua, type = script/lua
+    provides = hooks.linking.alsa-loopback.target.find-filter
+  }
+  {
+    name = linking/alsa-loopback/find-alsa-target.lua, type = script/lua
+    provides = hooks.linking.alsa-loopback.target.find-alsa
+  }
   {
     name = linking/prepare-link.lua, type = script/lua
     provides = hooks.linking.target.prepare-link
@@ -747,6 +755,8 @@ wireplumber.components = [
               hooks.linking.default.target.find-default,
               hooks.linking.default.target.find-best,
               hooks.linking.default.target.get-filter-from,
+              hooks.linking.alsa-loopback.target.find-filter,
+              hooks.linking.alsa-loopback.target.find-alsa,
               hooks.linking.pause-playback ]
   }
 

src/scripts/linking/alsa-loopback/find-alsa-target.lua

@@ -0,0 +1,102 @@
+-- WirePlumber
+--
+-- Copyright © 2025 Collabora Ltd.
+--
+-- SPDX-License-Identifier: MIT
+--
+-- Finds the hardware target for a linkable in 'alsa_loopback.*' collection
+
+lutils = require ("linking-utils")
+cutils = require ("common-utils")
+log = Log.open_topic ("s-linking")
+
+SimpleEventHook {
+  name = "linking/alsa-loopback/find-alsa-target",
+  before = "linking/prepare-link",
+  interests = {
+    EventInterest {
+      Constraint { "event.type", "=", "select-target" },
+      Constraint { "collection.name", "#", "alsa_loopback.*" },
+    },
+  },
+  execute = function (event)
+    local source, om, si, si_props, si_flags, target =
+        lutils:unwrap_select_target_event (event)
+
+    -- Bypass the hook if the target is already picked up
+    if target then
+      return
+    end
+
+    -- Get the collection name
+    local collection_name = si_props["collection.name"]
+    assert (collection_name)
+
+    local target_direction = cutils.getTargetDirection (si_props)
+    local target_picked = nil
+    local target_can_passthrough = false
+    local target_priority = 0
+
+    log:info (string.format ("handling item %d: %s (%s)", si.id,
+        si_props ["node.name"], si_props ["node.id"]))
+
+    -- Find the hardware target (session items without node.link-group property)
+    -- matching the collection name
+    for target in om:iterate {
+      type = "SiLinkable",
+      Constraint { "item.node.type", "=", "device" },
+      Constraint { "item.node.direction", "=", target_direction },
+      Constraint { "media.type", "=", si_props ["media.type"] },
+      Constraint { "collection.name", "=", collection_name },
+      Constraint { "node.link-group", "-" },
+    } do
+      local target_props = target.properties
+      local priority = tonumber (target_props ["priority.session"]) or 0
+
+      log:debug (string.format ("Looking at: %s (%s)",
+          target_props ["node.name"], target_props ["node.id"]))
+
+      if not lutils.canLink (si_props, target) then
+        log:debug ("... cannot link, skip linkable")
+        goto skip_linkable
+      end
+
+      local passthrough_compatible, can_passthrough =
+          lutils.checkPassthroughCompatibility (si, target)
+      if not passthrough_compatible then
+        log:debug ("... passthrough is not compatible, skip linkable")
+        goto skip_linkable
+      end
+
+      if target_picked == nil or priority > target_priority then
+        log:debug ("... picked")
+        target_picked = target
+        target_can_passthrough = can_passthrough
+        target_priority = priority
+      end
+
+      ::skip_linkable::
+    end
+
+    local can_passthrough, passthrough_compatible
+    if target then
+      passthrough_compatible, can_passthrough =
+          lutils.checkPassthroughCompatibility (si, target)
+      if lutils.canLink (si_props, target) and passthrough_compatible then
+        target_picked = true
+      end
+    end
+
+    if target_picked then
+      log:info (si,
+        string.format ("... hardware target picked: %s (%s), can_passthrough:%s",
+          target_picked.properties ["node.name"],
+          target_picked.properties ["node.id"],
+          tostring (target_can_passthrough)))
+      si_flags.can_passthrough = target_can_passthrough
+      event:set_data ("target", target_picked)
+    else
+      event:stop_processing ()
+    end
+  end
+}:register ()

src/scripts/linking/alsa-loopback/find-filter-target.lua

@@ -0,0 +1,97 @@
+-- WirePlumber
+--
+-- Copyright © 2026 Collabora Ltd.
+--
+-- SPDX-License-Identifier: MIT
+--
+-- Finds the filter target for a linkable in 'alsa_loopback.*' collection
+
+lutils = require ("linking-utils")
+cutils = require ("common-utils")
+log = Log.open_topic ("s-linking")
+
+SimpleEventHook {
+  name = "linking/alsa-loopback/find-filter-target",
+  before = "linking/alsa-loopback/find-alsa-target",
+  interests = {
+    EventInterest {
+      Constraint { "event.type", "=", "select-target" },
+      Constraint { "collection.name", "#", "alsa_loopback.*" },
+    },
+  },
+  execute = function (event)
+    local source, om, si, si_props, si_flags, target =
+        lutils:unwrap_select_target_event (event)
+
+    -- Bypass the hook if the target is already picked up
+    if target then
+      return
+    end
+
+    -- Get the collection name
+    local collection_name = si_props["collection.name"]
+    assert (collection_name)
+
+    local target_direction = cutils.getTargetDirection (si_props)
+    local target_picked = nil
+    local target_can_passthrough = false
+
+    log:info (string.format ("handling item %d: %s (%s)", si.id,
+        si_props ["node.name"], si_props ["node.id"]))
+
+    -- Find the first filter target (session items with node.link-group property
+    -- that is not an alsa loopback item) matching the collection name
+    for target in om:iterate {
+      type = "SiLinkable",
+      Constraint { "item.node.type", "=", "device" },
+      Constraint { "item.node.direction", "=", target_direction },
+      Constraint { "media.type", "=", si_props ["media.type"] },
+      Constraint { "collection.name", "=", collection_name },
+      Constraint { "node.link-group", "+" },
+      Constraint { "alsa.loopback.id", "-" },
+    } do
+      local target_props = target.properties
+
+      log:debug (string.format ("Looking at: %s (%s)",
+          target_props ["node.name"], target_props ["node.id"]))
+
+      if not lutils.canLink (si_props, target) then
+        log:debug ("... cannot link, skip linkable")
+        goto skip_linkable
+      end
+
+      local passthrough_compatible, can_passthrough =
+          lutils.checkPassthroughCompatibility (si, target)
+      if not passthrough_compatible then
+        log:debug ("... passthrough is not compatible, skip linkable")
+        goto skip_linkable
+      end
+
+      log:debug ("... picked")
+      target_picked = target
+      target_can_passthrough = can_passthrough
+      break
+
+      ::skip_linkable::
+    end
+
+    local can_passthrough, passthrough_compatible
+    if target then
+      passthrough_compatible, can_passthrough =
+          lutils.checkPassthroughCompatibility (si, target)
+      if lutils.canLink (si_props, target) and passthrough_compatible then
+        target_picked = true
+      end
+    end
+
+    if target_picked then
+      log:info (si,
+        string.format ("... filter target picked: %s (%s), can_passthrough:%s",
+          target_picked.properties ["node.name"],
+          target_picked.properties ["node.id"],
+          tostring (target_can_passthrough)))
+      si_flags.can_passthrough = target_can_passthrough
+      event:set_data ("target", target_picked)
+    end
+  end
+}:register ()