phy: serial: Automatically configure low latency on Linux

Use the `serialport_low_latency` [1] crate to automatically configure serial
ports for low latency if possible.  Unfortunately, the crate only
supports Linux for the time being - on other platforms, low latency has
to be configured manually by other means.

[1]: https://github.com/michaellass/serialport_low_latency

Cargo.toml

@@ -13,7 +13,7 @@ repository = "https://github.com/rahix/profirust"
 
 [features]
 phy-linux = ["rs485", "libc", "std"]
-phy-serial = ["serialport", "std"]
+phy-serial = ["serialport", "std", "serialport_low_latency"]
 phy-simulator = ["std"]
 phy-rp2040 = ["rp2040-hal", "fugit", "embedded-hal", "nb", "cortex-m"]
 std = ["managed/std"]
@@ -37,6 +37,9 @@ rp2040-hal = { version = "0.9.0", optional = true }
 rs485 = { version = "0.1.0", optional = true }
 serialport = { version = "4.6.0", optional = true }
 
+[target.'cfg(target_os = "linux")'.dependencies]
+serialport_low_latency = { version = "0.1.1", optional = true }
+
 [workspace]
 members = [
     "gsd-parser/",

src/phy/serial.rs

@@ -111,12 +111,15 @@ impl SerialPortPhy {
     }
 
     fn new_inner(serial_port: Cow<'_, str>, baudrate: crate::Baudrate) -> Self {
-        let port = serialport::new(serial_port, u32::try_from(baudrate.to_rate()).unwrap())
+        use serialport::SerialPort;
+
+        #[allow(unused_mut)]
+        let mut port = serialport::new(serial_port, u32::try_from(baudrate.to_rate()).unwrap())
             .data_bits(serialport::DataBits::Eight)
             .flow_control(serialport::FlowControl::None)
             .parity(serialport::Parity::Even)
             .stop_bits(serialport::StopBits::One)
-            .open()
+            .open_native()
             .unwrap();
 
         assert_eq!(
@@ -125,10 +128,13 @@ impl SerialPortPhy {
             "baudrate not configured correctly"
         );
 
+        #[cfg(target_os = "linux")]
+        serialport_low_latency::enable_low_latency(&mut port).unwrap();
+
         let buffer = crate::phy::BufferHandle::from(vec![0u8; 512]);
 
         Self {
-            port,
+            port: Box::new(port),
             data: PhyData::Rx { buffer, length: 0 },
             last_rx: None,
         }