From b7354d3bf1aa9d9d128277b48af4b1f29ddb2ec9 Mon Sep 17 00:00:00 2001
From: Christian Meusel <christian.meusel@posteo.de>
Date: Sat, 4 Jan 2025 22:04:29 +0100
Subject: [PATCH] Set DTR when opening a serial port by default

---
 src/lib.rs         | 23 +++++++++++++++++++++++
 src/posix/tty.rs   | 13 +++++++++++--
 src/windows/com.rs |  4 ++++
 3 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 79d0f377..158fdef1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -331,6 +331,8 @@ pub struct SerialPortBuilder {
     stop_bits: StopBits,
     /// Amount of time to wait to receive data before timing out
     timeout: Duration,
+    /// The state to set DTR to when opening the device
+    dtr_on_open: Option<bool>,
 }
 
 impl SerialPortBuilder {
@@ -393,6 +395,22 @@ impl SerialPortBuilder {
         self
     }
 
+    /// Set data terminal ready (DTR) to the given state when opening the device
+    #[must_use]
+    pub fn dtr_on_open(mut self, state: bool) -> Self {
+        self.dtr_on_open = Some(state);
+        self
+    }
+
+    /// Preserve the state of data terminal ready (DTR) when opening the device. Your outcome may
+    /// vary depending on the operation system. For example, Linux sets DTR by default and Windows
+    /// doesn't.
+    #[must_use]
+    pub fn preserve_dtr_on_open(mut self) -> Self {
+        self.dtr_on_open = None;
+        self
+    }
+
     /// Open a cross-platform interface to the port with the specified settings
     pub fn open(self) -> Result<Box<dyn SerialPort>> {
         #[cfg(unix)]
@@ -837,6 +855,11 @@ pub fn new<'a>(path: impl Into<std::borrow::Cow<'a, str>>, baud_rate: u32) -> Se
         parity: Parity::None,
         stop_bits: StopBits::One,
         timeout: Duration::from_millis(0),
+        // By default, set DTR when opening the device. There are USB devices performing "wait for
+        // DTR" before sending any data and users stumbled over this multiple times (see issues #29
+        // and #204). We are expecting little to no negative consequences from setting DTR by
+        // default but less hassle for users.
+        dtr_on_open: Some(true),
     }
 }
 
diff --git a/src/posix/tty.rs b/src/posix/tty.rs
index fe7e8499..47449d90 100644
--- a/src/posix/tty.rs
+++ b/src/posix/tty.rs
@@ -183,14 +183,23 @@ impl TTYPort {
         termios::set_termios(fd.0, &termios)?;
 
         // Return the final port object
-        Ok(TTYPort {
+        let mut port = TTYPort {
             fd: fd.into_raw(),
             timeout: builder.timeout,
             exclusive: true,
             port_name: Some(builder.path.clone()),
             #[cfg(any(target_os = "ios", target_os = "macos"))]
             baud_rate: builder.baud_rate,
-        })
+        };
+
+        // Ignore setting DTR for pseudo terminals (indicated by baud_rate == 0).
+        if builder.baud_rate > 0 {
+            if let Some(dtr) = builder.dtr_on_open {
+                port.write_data_terminal_ready(dtr)?;
+            }
+        }
+
+        Ok(port)
     }
 
     /// Returns the exclusivity of the port
diff --git a/src/windows/com.rs b/src/windows/com.rs
index fa1abbca..1bf368f6 100644
--- a/src/windows/com.rs
+++ b/src/windows/com.rs
@@ -85,6 +85,10 @@ impl COMPort {
         dcb::set_flow_control(&mut dcb, builder.flow_control);
         dcb::set_dcb(handle, dcb)?;
 
+        if let Some(dtr) = builder.dtr_on_open {
+            com.write_data_terminal_ready(dtr)?;
+        }
+
         com.set_timeout(builder.timeout)?;
         com.port_name = Some(builder.path.clone());
         Ok(com)