[feat](trx-backend-soapysdr): seed LNA gain from hardware at init

Add read_named_gain to IqSource (default: None) and implement it in
RealIqSource via Device::gain_element. Read the "LNA" element before
boxing the source so the initial sdr_lna_gain_db reflects the actual
hardware state, making the UI control visible and correct on first
connect. Devices without an LNA element (e.g. RTL-SDR with "TUNER")
return None and the control stays hidden.

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

@@ -63,6 +63,12 @@ pub trait IqSource: Send + 'static {
         Ok(())
     }
 
+    /// Read the current value of a named gain element. Returns `None` for
+    /// sources that do not support named gain elements.
+    fn read_named_gain(&self, _name: &str) -> Option<f64> {
+        None
+    }
+
     /// 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> {

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

@@ -10,6 +10,7 @@ pub mod vchan_impl;
 use std::pin::Pin;
 use std::sync::atomic::Ordering;
 use std::sync::{Arc, Mutex};
+use dsp::IqSource as _;
 use trx_core::radio::freq::{Band, Freq};
 use trx_core::rig::response::RigError;
 use trx_core::rig::state::{RigFilterState, SpectrumData, VchanRdsEntry, WfmDenoiseLevel};
@@ -155,13 +156,21 @@ impl SoapySdrRig {
         let hardware_center_hz = initial_freq.hz as i64 - center_offset_hz;
 
         // Create real IQ source from hardware device.
-        let iq_source: Box<dyn dsp::IqSource> = Box::new(real_iq_source::RealIqSource::new(
+        let iq_source = real_iq_source::RealIqSource::new(
             args,
             hardware_center_hz as f64,
             sdr_sample_rate as f64,
             bandwidth_hz as f64,
             effective_gain_db,
-        )?);
+        )?;
+        // Read the initial LNA gain from the hardware before the source is
+        // moved into the pipeline thread.  Returns None on devices that do
+        // not expose an "LNA" gain element (e.g. RTL-SDR exposes "TUNER").
+        let initial_lna_gain_db = iq_source.read_named_gain("LNA");
+        if let Some(lna) = initial_lna_gain_db {
+            tracing::info!("SDR LNA gain element present, initial value: {:.1} dB", lna);
+        }
+        let iq_source: Box<dyn dsp::IqSource> = Box::new(iq_source);
 
         let primary_channel_count = channels.len();
         let mut all_channels = channels.to_vec();
@@ -287,7 +296,7 @@ impl SoapySdrRig {
             wfm_denoise: WfmDenoiseLevel::Auto,
             gain_db,
             max_gain_db,
-            lna_gain_db: None,
+            lna_gain_db: initial_lna_gain_db,
             agc_enabled,
             squelch_enabled,
             squelch_threshold_db,

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

@@ -185,6 +185,12 @@ impl IqSource for RealIqSource {
             .map_err(|e| format!("Failed to set SDR {} gain: {}", name, e))
     }
 
+    fn read_named_gain(&self, name: &str) -> Option<f64> {
+        self.device
+            .gain_element(soapysdr::Direction::Rx, 0, name)
+            .ok()
+    }
+
     fn set_gain_mode(&mut self, automatic: bool) -> Result<(), String> {
         self.device
             .set_gain_mode(soapysdr::Direction::Rx, 0, automatic)