From 9fc79408dc97ba23dbd0ef2d71931f0aa2275c97 Mon Sep 17 00:00:00 2001
From: Ian Gulliver <github@flamingcow.io>
Date: Sun, 5 Apr 2026 20:47:48 +0900
Subject: [PATCH] Non-blocking USB TX with ring buffer and immediate drain

---
 picomap.cpp | 61 +++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 43 insertions(+), 18 deletions(-)

diff --git a/picomap.cpp b/picomap.cpp
index 16f73f3..d32693a 100644
--- a/picomap.cpp
+++ b/picomap.cpp
@@ -6,23 +6,47 @@
 
 #include "w6300.h"
 
-static void usb_write(const uint8_t* data, uint32_t len) {
-    while (len > 0) {
-        uint32_t avail = tud_cdc_write_available();
-        if (avail == 0) {
-            tud_task();
-            continue;
-        }
-        uint32_t chunk = len < avail ? len : avail;
-        tud_cdc_write(data, chunk);
-        tud_cdc_write_flush();
-        data += chunk;
-        len -= chunk;
-    }
-}
+struct tx_buf {
+    uint8_t data[512];
+    uint16_t head = 0;
+    uint16_t tail = 0;
 
-static void send_bytes(const std::vector<uint8_t> &data) {
-    usb_write(data.data(), data.size());
+    uint16_t used() const { return tail - head; }
+    uint16_t free() const { return sizeof(data) - used(); }
+    bool empty() const { return head == tail; }
+
+    void push(const uint8_t* src, uint16_t len) {
+        if (len > free()) return;
+        for (uint16_t i = 0; i < len; i++)
+            data[(tail + i) % sizeof(data)] = src[i];
+        tail += len;
+    }
+
+    void drain() {
+        while (!empty()) {
+            uint32_t avail = tud_cdc_write_available();
+            if (avail == 0) break;
+            uint16_t offset = head % sizeof(data);
+            uint16_t contig = sizeof(data) - offset;
+            uint16_t pending = used();
+            uint16_t chunk = pending < contig ? pending : contig;
+            if (chunk > avail) chunk = avail;
+            tud_cdc_write(data + offset, chunk);
+            head += chunk;
+        }
+        tud_cdc_write_flush();
+        if (head >= sizeof(data)) {
+            head -= sizeof(data);
+            tail -= sizeof(data);
+        }
+    }
+};
+
+static tx_buf usb_tx;
+
+static void send_bytes(const std::vector<uint8_t>& data) {
+    usb_tx.push(data.data(), data.size());
+    usb_tx.drain();
 }
 
 static bool w6300_init() {
@@ -49,14 +73,15 @@ static bool w6300_init() {
 int main() {
     tusb_init();
 
-    if (!w6300_init()) {
-    }
+    w6300_init();
 
     static static_vector<uint8_t, 256> rx_buf;
 
     while (true) {
         tud_task();
 
+        usb_tx.drain();
+
         while (tud_cdc_available()) {
             uint8_t byte;
             if (tud_cdc_read(&byte, 1) != 1) break;