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usart_async_buffered_generalized2/src/bin/main.rs

@@ -1,18 +1,16 @@
 // src/bin/main.rs
 #![no_std]
 #![no_main]
-
 use defmt::*;
 use embassy_executor::Spawner;
 use embassy_stm32::bind_interrupts;
 use embassy_stm32::peripherals;
 use embassy_stm32::usart::{BufferedInterruptHandler, BufferedUart, Config};
 use embedded_io_async::{Read, Write};
-use embassy_time::{Timer, Duration};
+use embassy_time::{Timer, Duration, Instant};
 use static_cell::StaticCell;
 use embassy_futures::yield_now;
 use {defmt_rtt as _, panic_probe as _};
-
 use embassy_futures::select::{select, Either};
 use embassy_sync::blocking_mutex::raw::CriticalSectionRawMutex;
 use embassy_sync::pipe::Pipe;
@@ -20,7 +18,16 @@ use embassy_sync::pipe::Pipe;
 const TX_PIPE_CAP: usize = 1024;
 const RX_PIPE_CAP: usize = 1024;
 
-// Two pipes: one for RX, one for TX.
+// ISR RX ring capacity = RX_BUF len
+const ISR_RX_BUF_CAP: usize = 256;
+
+// Yield 1/2 the time it takes to fill ISR RX ring.
+const YIELD_MARGIN_NUM: u32 = 1;
+const YIELD_MARGIN_DEN: u32 = 2;
+
+// Ensure RX_PIPE_CAP can hold this.
+const WORST_MAIN_LATENCY_MS: u32 = 20;
+
 static UART_TX: Pipe<CriticalSectionRawMutex, TX_PIPE_CAP> = Pipe::new();
 static UART_RX: Pipe<CriticalSectionRawMutex, RX_PIPE_CAP> = Pipe::new();
 
@@ -34,7 +41,6 @@ bind_interrupts!(
 async fn uart_task(mut uart: BufferedUart<'static>) {
     let mut rx_byte = [0u8; 1];
     let mut tx_buf = [0u8; 64];
-
     loop {
         // Wait for either RX or TX events.
         let rx_fut = uart.read(&mut rx_byte);
@@ -43,17 +49,15 @@ async fn uart_task(mut uart: BufferedUart<'static>) {
             let n = UART_TX.read(&mut tx_buf).await;
             n
         };
-
         match select(rx_fut, tx_fut).await {
             // Incoming data from UART hardware
             Either::First(res) => {
                 if let Ok(_) = res {
-                    // Forward to RX pipe (non-blocking)
-                    let _ = UART_RX.try_write(&rx_byte);
+                    // Forward to RX pipe
+                    let _ = UART_RX.write(&rx_byte).await;
                     let _ = UART_TX.try_write(&rx_byte);
                 }
             }
-
             // Outgoing data waiting in TX pipe
             Either::Second(n) => {
                 unwrap!(uart.write(&tx_buf[..n]).await);
@@ -62,19 +66,57 @@ async fn uart_task(mut uart: BufferedUart<'static>) {
     }
 }
 
+fn preflight_and_suggest_yield_period(baud: u32) -> Duration {
+    // Approx bytes per second for 8N1 (10 bits per byte on the wire)
+    let bytes_per_sec = (baud / 10).max(1);
+
+    // Time until ISR RX ring fills, in microseconds.
+    let t_fill_us = (ISR_RX_BUF_CAP as u64) * 1_000_000u64 / (bytes_per_sec as u64);
+
+    // Choose a yield period as a fraction of t_fill.
+    let yield_us = (t_fill_us as u64)
+        .saturating_mul(YIELD_MARGIN_NUM as u64)
+        / (YIELD_MARGIN_DEN as u64);
+
+    // Verify RX pipe can absorb a worst-case app latency so uart_task
+    // can always forward without dropping when it runs.
+    let required_rx_pipe = (bytes_per_sec as u64) * (WORST_MAIN_LATENCY_MS as u64) / 1000;
+
+    if (RX_PIPE_CAP as u64) < required_rx_pipe {
+        core::panic!(
+            "RX pipe too small: have {}B, need >= {}B for {}ms at {} bps",
+            RX_PIPE_CAP, required_rx_pipe, WORST_MAIN_LATENCY_MS, baud
+        );
+    }
+
+    info!(
+        "Preflight: baud={}, rx_isr={}B, rx_pipe={}B, bytes/s={}, t_fill_us={}, yield_us={}",
+        baud,
+        ISR_RX_BUF_CAP,
+        RX_PIPE_CAP,
+        bytes_per_sec,
+        t_fill_us,
+        yield_us
+    );
+
+    // Never choose zero.
+    Duration::from_micros(yield_us.max(1) as u64)
+}
+
 #[embassy_executor::main]
 async fn main(spawner: Spawner) {
     info!("tititititi");
     let p = embassy_stm32::init(Default::default());
-
     static TX_BUF: StaticCell<[u8; 256]> = StaticCell::new();
     static RX_BUF: StaticCell<[u8; 256]> = StaticCell::new();
     let tx_buf = TX_BUF.init([0; 256]);
     let rx_buf = RX_BUF.init([0; 256]);
-
     let mut cfg = Config::default();
     cfg.baudrate = 230_400;
 
+    // Call preflight and get the computed yield period
+    let yield_period = preflight_and_suggest_yield_period(cfg.baudrate);
+
     let usart = BufferedUart::new(
         p.USART1,
         p.PA10, // RX
@@ -84,24 +126,15 @@ async fn main(spawner: Spawner) {
         Irqs,
         cfg,
     ).unwrap();
-
     info!("starting uart task");
     spawner.spawn(uart_task(usart)).unwrap();
 
     let mut counter: u32 = 0;
     let mut rx_buf = [0u8; 64];
+    let mut last_yield = Instant::now();
 
     loop {
         counter = counter.wrapping_add(1);
-
-        if counter % 10000 == 0 {
-            info!("CPU busy {}", counter);
-        }
-        if counter % 100000 == 0 {
-            let _ = UART_TX.try_write(b"Hello\r\n");
-            info!("Queued Hello");
-        }
-
         // Poll RX pipe for new data (non-blocking)
         if let Ok(n) = UART_RX.try_read(&mut rx_buf) {
             if n > 0 {
@@ -109,10 +142,13 @@ async fn main(spawner: Spawner) {
             }
         }
 
-        if counter % 100000 == 0 {
+        // Guaranteed to yield before ISR RX buffer can overflow
+        if Instant::now().duration_since(last_yield) >= yield_period {
             yield_now().await;
+            last_yield = Instant::now();
+            info!("Yield mf {}", counter);
         }
         // Timer::after(Duration::from_micros(1)).await;
-        Timer::after(Duration::from_secs(5)).await;
+        // Timer::after(Duration::from_secs(5)).await;
     }
 }