From 500a78cd6862c4b17e38795fecfc12749bacbaad Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Dav=C3=AD=C3=B0=20Steinn=20Geirsson?= <david@dsg.is>
Date: Tue, 17 Mar 2026 19:20:55 +0000
Subject: [PATCH] sound: dynamically construct streams/chmaps based on
 --streams flag

Also fixes existing bug where chmaps count was hard-coded to 1
despite 2 chmap entries being created.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 vhost-device-sound/src/device.rs | 97 ++++++++++++++++++++++++--------
 1 file changed, 75 insertions(+), 22 deletions(-)

diff --git a/vhost-device-sound/src/device.rs b/vhost-device-sound/src/device.rs
index 915661a..90dc97f 100644
--- a/vhost-device-sound/src/device.rs
+++ b/vhost-device-sound/src/device.rs
@@ -493,39 +493,50 @@ pub struct VhostUserSoundBackend {
 
 impl VhostUserSoundBackend {
     pub fn new(config: SoundConfig) -> Result<Self> {
-        let streams = vec![
-            Stream {
-                id: 0,
-                direction: Direction::Output,
-                ..Stream::default()
-            },
-            Stream {
-                id: 1,
-                direction: Direction::Input,
-                ..Stream::default()
-            },
-        ];
-        let streams_no = streams.len();
-        let streams = Arc::new(RwLock::new(streams));
-        let jacks: Arc<RwLock<Vec<VirtioSoundJackInfo>>> = Arc::new(RwLock::new(Vec::new()));
+        let mut streams = Vec::new();
+        let mut chmaps_info: Vec<VirtioSoundChmapInfo> = Vec::new();
+
         let mut positions = [VIRTIO_SND_CHMAP_NONE; VIRTIO_SND_CHMAP_MAX_SIZE];
         positions[0] = VIRTIO_SND_CHMAP_FL;
         positions[1] = VIRTIO_SND_CHMAP_FR;
-        let chmaps_info: Vec<VirtioSoundChmapInfo> = vec![
-            VirtioSoundChmapInfo {
+
+        if config.has_output() {
+            streams.push(Stream {
+                id: streams.len(),
+                direction: Direction::Output,
+                ..Stream::default()
+            });
+            chmaps_info.push(VirtioSoundChmapInfo {
                 direction: VIRTIO_SND_D_OUTPUT,
                 channels: 2,
                 positions,
                 ..VirtioSoundChmapInfo::default()
-            },
-            VirtioSoundChmapInfo {
+            });
+        }
+
+        if config.has_input() {
+            streams.push(Stream {
+                id: streams.len(),
+                direction: Direction::Input,
+                ..Stream::default()
+            });
+            chmaps_info.push(VirtioSoundChmapInfo {
                 direction: VIRTIO_SND_D_INPUT,
                 channels: 2,
                 positions,
                 ..VirtioSoundChmapInfo::default()
-            },
-        ];
+            });
+        }
+
+        let chmaps_no = chmaps_info.len();
+        let streams_no = streams.len();
+        let streams = Arc::new(RwLock::new(streams));
+        let jacks: Arc<RwLock<Vec<VirtioSoundJackInfo>>> = Arc::new(RwLock::new(Vec::new()));
         let chmaps: Arc<RwLock<Vec<VirtioSoundChmapInfo>>> = Arc::new(RwLock::new(chmaps_info));
+
+        if streams_no == 0 {
+            return Err(Error::UnexpectedAudioBackendConfiguration);
+        }
         log::trace!("VhostUserSoundBackend::new(config = {:?})", &config);
         let threads = if config.multi_thread {
             vec![
@@ -576,7 +587,7 @@ impl VhostUserSoundBackend {
             virtio_cfg: VirtioSoundConfig {
                 jacks: 0.into(),
                 streams: Le32::from(streams_no as u32),
-                chmaps: 1.into(),
+                chmaps: Le32::from(chmaps_no as u32),
                 controls: 0.into(),
             },
             exit_consumer,
@@ -1066,4 +1077,46 @@ mod tests {
 
         test_dir.close().unwrap();
     }
+
+    #[test]
+    fn test_sound_backend_output_only() {
+        crate::init_logger();
+        let config = SoundConfig::new(false, BackendType::Null, true, false);
+        let backend = VhostUserSoundBackend::new(config).expect("Could not create backend.");
+
+        // VirtioSoundConfig: jacks(4) + streams(4) + chmaps(4) + controls(4) = 16 bytes
+        let cfg = backend.get_config(0, 16);
+        assert_eq!(cfg.len(), 16);
+        // streams is at offset 4, little-endian u32
+        let streams = u32::from_le_bytes([cfg[4], cfg[5], cfg[6], cfg[7]]);
+        let chmaps = u32::from_le_bytes([cfg[8], cfg[9], cfg[10], cfg[11]]);
+        assert_eq!(streams, 1);
+        assert_eq!(chmaps, 1);
+    }
+
+    #[test]
+    fn test_sound_backend_input_only() {
+        crate::init_logger();
+        let config = SoundConfig::new(false, BackendType::Null, false, true);
+        let backend = VhostUserSoundBackend::new(config).expect("Could not create backend.");
+
+        let cfg = backend.get_config(0, 16);
+        let streams = u32::from_le_bytes([cfg[4], cfg[5], cfg[6], cfg[7]]);
+        let chmaps = u32::from_le_bytes([cfg[8], cfg[9], cfg[10], cfg[11]]);
+        assert_eq!(streams, 1);
+        assert_eq!(chmaps, 1);
+    }
+
+    #[test]
+    fn test_sound_backend_both_streams() {
+        crate::init_logger();
+        let config = SoundConfig::new(false, BackendType::Null, true, true);
+        let backend = VhostUserSoundBackend::new(config).expect("Could not create backend.");
+
+        let cfg = backend.get_config(0, 16);
+        let streams = u32::from_le_bytes([cfg[4], cfg[5], cfg[6], cfg[7]]);
+        let chmaps = u32::from_le_bytes([cfg[8], cfg[9], cfg[10], cfg[11]]);
+        assert_eq!(streams, 2);
+        assert_eq!(chmaps, 2);  // Also verifies the chmaps bug fix (was hard-coded to 1)
+    }
 }