[feat](trx-backend-soapysdr): implement LNA gain element control

Add set_named_gain to the IqSource trait and implement it in
RealIqSource via soapysdr Device::set_gain_element. Wire a
lna_gain_cmd channel through SdrPipeline so the IQ read loop applies
LNA gain changes on the next iteration. Add set_sdr_lna_gain to
SoapySdrRig and expose the current value via filter_state.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-server/trx-backend/trx-backend-soapysdr/src/dsp.rs

@@ -57,6 +57,12 @@ pub trait IqSource: Send + 'static {
         Ok(())
     }
 
+    /// Apply a new gain to a named gain element (e.g. "LNA"). Default
+    /// implementation is a no-op for sources that do not support named gains.
+    fn set_named_gain(&mut self, _name: &str, _gain_db: f64) -> Result<(), String> {
+        Ok(())
+    }
+
     /// Enable or disable hardware automatic gain control.  Default
     /// implementation is a no-op for sources that do not support AGC.
     fn set_gain_mode(&mut self, _automatic: bool) -> Result<(), String> {
@@ -106,6 +112,9 @@ pub struct SdrPipeline {
     /// Write `Some(gain_db)` here to adjust the hardware RX gain.
     /// The IQ read loop picks it up on the next iteration.
     pub gain_cmd: Arc<std::sync::Mutex<Option<f64>>>,
+    /// Write `Some(gain_db)` here to adjust the LNA gain element.
+    /// The IQ read loop picks it up on the next iteration.
+    pub lna_gain_cmd: Arc<std::sync::Mutex<Option<f64>>>,
     /// Write `Some(enabled)` here to switch hardware AGC on or off.
     /// The IQ read loop picks it up on the next iteration.
     pub agc_cmd: Arc<std::sync::Mutex<Option<bool>>>,
@@ -183,6 +192,9 @@ impl SdrPipeline {
         let thread_retune_cmd = retune_cmd.clone();
         let gain_cmd: Arc<std::sync::Mutex<Option<f64>>> = Arc::new(std::sync::Mutex::new(None));
         let thread_gain_cmd = gain_cmd.clone();
+        let lna_gain_cmd: Arc<std::sync::Mutex<Option<f64>>> =
+            Arc::new(std::sync::Mutex::new(None));
+        let thread_lna_gain_cmd = lna_gain_cmd.clone();
         let agc_cmd: Arc<std::sync::Mutex<Option<bool>>> = Arc::new(std::sync::Mutex::new(None));
         let thread_agc_cmd = agc_cmd.clone();
 
@@ -197,6 +209,7 @@ impl SdrPipeline {
                     thread_spectrum_buf,
                     thread_retune_cmd,
                     thread_gain_cmd,
+                    thread_lna_gain_cmd,
                     thread_agc_cmd,
                 );
             })
@@ -210,6 +223,7 @@ impl SdrPipeline {
             sdr_sample_rate,
             retune_cmd,
             gain_cmd,
+            lna_gain_cmd,
             agc_cmd,
             shared_center_hz: Arc::new(AtomicI64::new(0)),
             audio_sample_rate,
@@ -290,6 +304,7 @@ fn iq_read_loop(
     spectrum_buf: Arc<Mutex<Option<Vec<f32>>>>,
     retune_cmd: Arc<std::sync::Mutex<Option<f64>>>,
     gain_cmd: Arc<std::sync::Mutex<Option<f64>>>,
+    lna_gain_cmd: Arc<std::sync::Mutex<Option<f64>>>,
     agc_cmd: Arc<std::sync::Mutex<Option<bool>>>,
 ) {
     let mut block = vec![Complex::new(0.0_f32, 0.0_f32); IQ_BLOCK_SIZE];
@@ -323,6 +338,15 @@ fn iq_read_loop(
                 }
             }
         }
+        if let Ok(mut cmd) = lna_gain_cmd.try_lock() {
+            if let Some(gain_db) = cmd.take() {
+                if let Err(e) = source.set_named_gain("LNA", gain_db) {
+                    tracing::warn!("SDR LNA gain change to {:.1} dB failed: {}", gain_db, e);
+                } else {
+                    tracing::info!("SDR LNA gain updated to {:.1} dB", gain_db);
+                }
+            }
+        }
         if let Ok(mut cmd) = agc_cmd.try_lock() {
             if let Some(enabled) = cmd.take() {
                 if let Err(e) = source.set_gain_mode(enabled) {

src/trx-server/trx-backend/trx-backend-soapysdr/src/lib.rs

@@ -52,6 +52,8 @@ pub struct SoapySdrRig {
     gain_db: f64,
     /// Optional hard ceiling for the applied hardware gain in dB.
     max_gain_db: Option<f64>,
+    /// Requested LNA gain element setting in dB (None if not set by user).
+    lna_gain_db: Option<f64>,
     /// Whether hardware AGC is currently enabled.
     agc_enabled: bool,
     /// Whether software squelch is enabled on primary channel (except WFM mode).
@@ -285,6 +287,7 @@ impl SoapySdrRig {
             wfm_denoise: WfmDenoiseLevel::Auto,
             gain_db,
             max_gain_db,
+            lna_gain_db: None,
             agc_enabled,
             squelch_enabled,
             squelch_threshold_db,
@@ -579,6 +582,25 @@ impl RigCat for SoapySdrRig {
         })
     }
 
+    fn set_sdr_lna_gain<'a>(
+        &'a mut self,
+        gain_db: f64,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            if !gain_db.is_finite() {
+                return Err("LNA gain must be finite".into());
+            }
+            if gain_db < 0.0 {
+                return Err("LNA gain must be >= 0".into());
+            }
+            self.lna_gain_db = Some(gain_db);
+            if let Ok(mut cmd) = self.pipeline.lna_gain_cmd.lock() {
+                *cmd = Some(gain_db);
+            }
+            Ok(())
+        })
+    }
+
     fn set_sdr_agc<'a>(
         &'a mut self,
         enabled: bool,
@@ -803,6 +825,7 @@ impl RigCat for SoapySdrRig {
                     .map(|max_gain| self.gain_db.min(max_gain))
                     .unwrap_or(self.gain_db),
             ),
+            sdr_lna_gain_db: self.lna_gain_db,
             sdr_agc_enabled: Some(self.agc_enabled),
             sdr_squelch_enabled: Some(self.squelch_enabled),
             sdr_squelch_threshold_db: Some(self.squelch_threshold_db as f64),

src/trx-server/trx-backend/trx-backend-soapysdr/src/real_iq_source.rs

@@ -179,6 +179,12 @@ impl IqSource for RealIqSource {
             .map_err(|e| format!("Failed to set SDR gain: {}", e))
     }
 
+    fn set_named_gain(&mut self, name: &str, gain_db: f64) -> Result<(), String> {
+        self.device
+            .set_gain_element(soapysdr::Direction::Rx, 0, name, gain_db)
+            .map_err(|e| format!("Failed to set SDR {} gain: {}", name, e))
+    }
+
     fn set_gain_mode(&mut self, automatic: bool) -> Result<(), String> {
         self.device
             .set_gain_mode(soapysdr::Direction::Rx, 0, automatic)