[feat](trx-backend-soapysdr): add crate scaffold with RigCat stub

Introduces the trx-backend-soapysdr crate with a compilable SoapySdrRig
struct that satisfies the Rig + RigCat trait bounds.  RX methods
(get_status, set_freq, set_mode, get_signal_strength) are implemented;
TX-only methods return RigError::not_supported.  as_audio_source returns
None for now (overridden in SDR-07).  Wires the crate into the workspace
and trx-backend (feature "soapysdr"), and fixes the non-exhaustive match
on RigAccess::Sdr in trx-server main.rs and rig_task.rs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-server/src/main.rs

@@ -290,6 +290,12 @@ async fn main() -> DynResult<()> {
                 resolved.rig, addr
             );
         }
+        RigAccess::Sdr { args } => {
+            info!(
+                "Starting trx-server (rig: {}, access: sdr {})",
+                resolved.rig, args
+            );
+        }
     }
 
     if let Some(ref cs) = resolved.callsign {

src/trx-server/src/rig_task.rs

@@ -91,6 +91,7 @@ pub async fn run_rig_task(
     match &config.access {
         RigAccess::Serial { path, baud } => info!("Serial: {} @ {} baud", path, baud),
         RigAccess::Tcp { addr } => info!("TCP CAT: {}", addr),
+        RigAccess::Sdr { args } => info!("SDR: {}", args),
     }
 
     let mut rig: Box<dyn RigCat> = config

src/trx-server/trx-backend/Cargo.toml

@@ -11,12 +11,13 @@ edition = "2021"
 default = ["ft817", "ft450d"]
 ft817 = ["dep:trx-backend-ft817"]
 ft450d = ["dep:trx-backend-ft450d"]
-soapysdr = []   # implementation wired in SDR-04
+soapysdr = ["dep:trx-backend-soapysdr"]
 
 [dependencies]
 trx-core = { path = "../../trx-core" }
 trx-backend-ft817 = { path = "trx-backend-ft817", optional = true }
 trx-backend-ft450d = { path = "trx-backend-ft450d", optional = true }
+trx-backend-soapysdr = { path = "./trx-backend-soapysdr", optional = true }
 tokio = { workspace = true, features = ["full"] }
 tokio-serial = { workspace = true }
 serde = { workspace = true, features = ["derive"] }

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

@@ -124,10 +124,7 @@ fn ft450d_factory(access: RigAccess) -> DynResult<Box<dyn RigCat>> {
 #[cfg(feature = "soapysdr")]
 fn soapysdr_factory(access: RigAccess) -> DynResult<Box<dyn RigCat>> {
     match access {
-        RigAccess::Sdr { args } => {
-            // trx_backend_soapysdr will be wired in once SDR-04 lands
-            Err(format!("soapysdr backend not yet implemented (args: {args})").into())
-        }
+        RigAccess::Sdr { args } => Ok(Box::new(trx_backend_soapysdr::SoapySdrRig::new(&args)?)),
         _ => Err("soapysdr backend requires Sdr access type".into()),
     }
 }

src/trx-server/trx-backend/trx-backend-soapysdr/Cargo.toml

@@ -0,0 +1,22 @@
+# SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+[package]
+name = "trx-backend-soapysdr"
+version = "0.1.0"
+edition = "2021"
+license = "BSD-2-Clause"
+
+[dependencies]
+trx-core = { path = "../../../trx-core" }
+tokio = { workspace = true, features = ["sync", "rt"] }
+serde = { workspace = true }
+tracing = { workspace = true }
+num-complex = "0.4"
+# soapysdr is an optional system-library dep gated behind a feature
+soapysdr = { version = "0.3", optional = true }
+
+[features]
+default = []
+soapysdr-sys = ["dep:soapysdr"]

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

@@ -0,0 +1,208 @@
+// SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+//
+// SPDX-License-Identifier: BSD-2-Clause
+
+use std::pin::Pin;
+
+use trx_core::radio::freq::{Band, Freq};
+use trx_core::rig::{
+    Rig, RigAccessMethod, RigCapabilities, RigCat, RigInfo, RigStatusFuture,
+};
+use trx_core::rig::response::RigError;
+use trx_core::{DynResult, RigMode};
+
+/// RX-only backend for any SoapySDR-compatible device.
+pub struct SoapySdrRig {
+    info: RigInfo,
+    freq: Freq,
+    mode: RigMode,
+}
+
+impl SoapySdrRig {
+    /// Construct a new `SoapySdrRig` from a SoapySDR device args string.
+    ///
+    /// The `args` value follows SoapySDR's key=value comma-separated convention
+    /// (e.g. `"driver=rtlsdr"` or `"driver=airspy,serial=00000001"`).
+    pub fn new(args: &str) -> DynResult<Self> {
+        tracing::info!("initialising SoapySDR backend (args={:?})", args);
+
+        let info = RigInfo {
+            manufacturer: "SoapySDR".to_string(),
+            model: "Generic SDR".to_string(),
+            revision: "".to_string(),
+            capabilities: RigCapabilities {
+                min_freq_step_hz: 1,
+                // Broad RX-only coverage: DC through 6 GHz as a single band.
+                supported_bands: vec![Band {
+                    low_hz: 0,
+                    high_hz: 6_000_000_000,
+                    tx_allowed: false,
+                }],
+                supported_modes: vec![
+                    RigMode::LSB,
+                    RigMode::USB,
+                    RigMode::CW,
+                    RigMode::CWR,
+                    RigMode::AM,
+                    RigMode::WFM,
+                    RigMode::FM,
+                    RigMode::DIG,
+                    RigMode::PKT,
+                ],
+                num_vfos: 1,
+                lockable: false,
+                attenuator: false,
+                preamp: false,
+                rit: false,
+                rpt: false,
+                split: false,
+                lock: false,
+            },
+            // There is no serial/TCP access for SDR devices; use a dummy TCP
+            // placeholder so `RigAccessMethod` (which has no SDR variant) can
+            // still carry the args string in a human-readable form.
+            access: RigAccessMethod::Tcp {
+                addr: format!("soapysdr:{}", args),
+            },
+        };
+
+        Ok(Self {
+            info,
+            freq: Freq { hz: 14_074_000 },
+            mode: RigMode::USB,
+        })
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Rig
+// ---------------------------------------------------------------------------
+
+impl Rig for SoapySdrRig {
+    fn info(&self) -> &RigInfo {
+        &self.info
+    }
+}
+
+// ---------------------------------------------------------------------------
+// RigCat
+// ---------------------------------------------------------------------------
+
+impl RigCat for SoapySdrRig {
+    // -- Supported RX methods -----------------------------------------------
+
+    fn get_status<'a>(&'a mut self) -> RigStatusFuture<'a> {
+        Box::pin(async move { Ok((self.freq, self.mode.clone(), None)) })
+    }
+
+    fn set_freq<'a>(
+        &'a mut self,
+        freq: Freq,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            tracing::debug!("SoapySdrRig: set_freq -> {} Hz", freq.hz);
+            self.freq = freq;
+            Ok(())
+        })
+    }
+
+    fn set_mode<'a>(
+        &'a mut self,
+        mode: RigMode,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            tracing::debug!("SoapySdrRig: set_mode -> {:?}", mode);
+            self.mode = mode;
+            Ok(())
+        })
+    }
+
+    fn get_signal_strength<'a>(
+        &'a mut self,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<u8>> + Send + 'a>> {
+        // RSSI mapping will be implemented in SDR-07; return 0 for now.
+        Box::pin(async move { Ok(0) })
+    }
+
+    // -- TX / unsupported methods -------------------------------------------
+
+    fn set_ptt<'a>(
+        &'a mut self,
+        _ptt: bool,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            Err(Box::new(RigError::not_supported("set_ptt")) as Box<dyn std::error::Error + Send + Sync>)
+        })
+    }
+
+    fn power_on<'a>(
+        &'a mut self,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            Err(Box::new(RigError::not_supported("power_on")) as Box<dyn std::error::Error + Send + Sync>)
+        })
+    }
+
+    fn power_off<'a>(
+        &'a mut self,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            Err(Box::new(RigError::not_supported("power_off")) as Box<dyn std::error::Error + Send + Sync>)
+        })
+    }
+
+    fn get_tx_power<'a>(
+        &'a mut self,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<u8>> + Send + 'a>> {
+        Box::pin(async move {
+            Err(Box::new(RigError::not_supported("get_tx_power")) as Box<dyn std::error::Error + Send + Sync>)
+        })
+    }
+
+    fn get_tx_limit<'a>(
+        &'a mut self,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<u8>> + Send + 'a>> {
+        Box::pin(async move {
+            Err(Box::new(RigError::not_supported("get_tx_limit")) as Box<dyn std::error::Error + Send + Sync>)
+        })
+    }
+
+    fn set_tx_limit<'a>(
+        &'a mut self,
+        _limit: u8,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            Err(Box::new(RigError::not_supported("set_tx_limit")) as Box<dyn std::error::Error + Send + Sync>)
+        })
+    }
+
+    fn toggle_vfo<'a>(
+        &'a mut self,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            Err(Box::new(RigError::not_supported("toggle_vfo")) as Box<dyn std::error::Error + Send + Sync>)
+        })
+    }
+
+    fn lock<'a>(
+        &'a mut self,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            Err(Box::new(RigError::not_supported("lock")) as Box<dyn std::error::Error + Send + Sync>)
+        })
+    }
+
+    fn unlock<'a>(
+        &'a mut self,
+    ) -> Pin<Box<dyn std::future::Future<Output = DynResult<()>> + Send + 'a>> {
+        Box::pin(async move {
+            Err(Box::new(RigError::not_supported("unlock")) as Box<dyn std::error::Error + Send + Sync>)
+        })
+    }
+
+    /// Returns `None` for now; will be overridden with `Some(self)` in SDR-07
+    /// once the IQ DSP pipeline is in place.
+    fn as_audio_source(&self) -> Option<&dyn trx_core::rig::AudioSource> {
+        None
+    }
+}