From 24dcf5d5bd0beac3d4785a618c8f9c86840031d1 Mon Sep 17 00:00:00 2001
From: taitep <taitep@taitep.se>
Date: Sun, 21 Dec 2025 21:25:29 +0100
Subject: [PATCH] Improve UART by using nonblocking stdin

---
 Cargo.lock        | 31 ++++++++++++++++++++++++++++++
 Cargo.toml        |  1 +
 src/basic_uart.rs | 48 ++++++++++++++++++++++-------------------------
 3 files changed, 54 insertions(+), 26 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index a77c839..8ff751b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -8,6 +8,24 @@ version = "1.0.100"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a23eb6b1614318a8071c9b2521f36b424b2c83db5eb3a0fead4a6c0809af6e61"
 
+[[package]]
+name = "bitflags"
+version = "2.10.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "812e12b5285cc515a9c72a5c1d3b6d46a19dac5acfef5265968c166106e31dd3"
+
+[[package]]
+name = "cfg-if"
+version = "1.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
+
+[[package]]
+name = "cfg_aliases"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "613afe47fcd5fac7ccf1db93babcb082c5994d996f20b8b159f2ad1658eb5724"
+
 [[package]]
 name = "libc"
 version = "0.2.176"
@@ -23,10 +41,23 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "nix"
+version = "0.30.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "74523f3a35e05aba87a1d978330aef40f67b0304ac79c1c00b294c9830543db6"
+dependencies = [
+ "bitflags",
+ "cfg-if",
+ "cfg_aliases",
+ "libc",
+]
+
 [[package]]
 name = "trve"
 version = "0.0.0"
 dependencies = [
  "anyhow",
  "memmap2",
+ "nix",
 ]
diff --git a/Cargo.toml b/Cargo.toml
index 003b0b5..c4c3ece 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -6,3 +6,4 @@ edition = "2024"
 [dependencies]
 anyhow = "1.0.100"
 memmap2 = "0.9.8"
+nix = { version = "0.30.1", features = ["fs"] }
diff --git a/src/basic_uart.rs b/src/basic_uart.rs
index 3c9e064..f178574 100644
--- a/src/basic_uart.rs
+++ b/src/basic_uart.rs
@@ -1,30 +1,35 @@
 use std::collections::VecDeque;
-use std::io::{Read, stdin};
+use std::io;
+use std::os::fd::AsFd;
 use std::sync::{Arc, Mutex};
 use std::thread;
 use std::time::Duration;
 
+use nix::fcntl::fcntl;
+use nix::fcntl::{FcntlArg, OFlag};
 use trve::consts::{Byte, DWord, HWord, Word};
 use trve::mem::{MemAccessFault, MemDeviceInterface, PageNum};
 
 /// byte 0: rx/tx
 /// byte 1: status (------rt, r=rxready, t=txready)/none
 pub struct BasicUart {
-    buffers: Mutex<UartBuffers>,
-}
-
-struct UartBuffers {
-    rx: VecDeque<u8>,
-    tx: VecDeque<u8>,
+    rx_buf: Mutex<VecDeque<u8>>,
 }
 
 impl BasicUart {
     pub fn new() -> Self {
+        // Make sure stdin is nonblocking
+        let stdin = io::stdin();
+        let fd = stdin.as_fd();
+        let flags = fcntl(fd, FcntlArg::F_GETFL).unwrap();
+        fcntl(
+            fd,
+            FcntlArg::F_SETFL(OFlag::from_bits_truncate(flags) | OFlag::O_NONBLOCK),
+        )
+        .unwrap();
+
         BasicUart {
-            buffers: Mutex::new(UartBuffers {
-                rx: VecDeque::new(),
-                tx: VecDeque::new(),
-            }),
+            rx_buf: Mutex::new(VecDeque::new()),
         }
     }
 
@@ -44,32 +49,23 @@ impl BasicUart {
     }
 
     fn write(&self, byte: u8) {
-        let mut bufs = self.buffers.lock().unwrap();
-        bufs.tx.push_back(byte);
+        print!("{}", byte as char);
     }
 
     fn read(&self) -> u8 {
-        let mut bufs = self.buffers.lock().unwrap();
-        bufs.rx.pop_front().unwrap_or(0)
+        self.rx_buf.lock().unwrap().pop_front().unwrap_or(0)
     }
 
     fn can_read(&self) -> bool {
-        let bufs = self.buffers.lock().unwrap();
-        !bufs.rx.is_empty()
+        !self.rx_buf.lock().unwrap().is_empty()
     }
 
     pub fn poll(&self) {
-        let mut bufs = self.buffers.lock().unwrap();
-
-        while let Some(byte) = bufs.tx.pop_front() {
-            print!("{}", byte as char);
-        }
+        let mut rx_buf = self.rx_buf.lock().unwrap();
 
         let mut buffer = [0u8; 1];
-        if let Ok(n) = stdin().read(&mut buffer) {
-            if n > 0 {
-                bufs.rx.push_back(buffer[0]);
-            }
+        while let Ok(1) = nix::unistd::read(io::stdin().as_fd(), &mut buffer) {
+            rx_buf.push_back(buffer[0]);
         }
     }
 }