From 76eb8f697f43b4777dffa66a327024b027a46d0e Mon Sep 17 00:00:00 2001
From: Keith Burzinski <kbx81x@gmail.com>
Date: Wed, 22 Apr 2026 16:16:14 -0500
Subject: [PATCH] [usb_uart] Derive TX output chunk count from `buffer_size`
 config (#15909)

---
 esphome/components/usb_uart/__init__.py | 13 ++++++++++++-
 esphome/components/usb_uart/usb_uart.h  |  5 +++--
 esphome/core/defines.h                  |  1 +
 3 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/esphome/components/usb_uart/__init__.py b/esphome/components/usb_uart/__init__.py
index 0e8994a3ed..d542788fb9 100644
--- a/esphome/components/usb_uart/__init__.py
+++ b/esphome/components/usb_uart/__init__.py
@@ -116,12 +116,23 @@ CONFIG_SCHEMA = cv.ensure_list(
 
 
 async def to_code(config):
+    # The output chunk pool/queue are compile-time-sized templates shared by all
+    # USBUartChannel instances, so use the largest buffer_size across every channel
+    # of every device.  Each chunk is 64 bytes (USB FS MPS); add one extra slot
+    # because LockFreeQueue<T,N> is a ring buffer that wastes one entry.
+    max_buffer_size = max(
+        channel[CONF_BUFFER_SIZE]
+        for device in config
+        for channel in device[CONF_CHANNELS]
+    )
+    output_chunk_count = max_buffer_size // 64 + 1
+    cg.add_define("USB_UART_OUTPUT_CHUNK_COUNT", output_chunk_count)
+
     for device in config:
         var = await register_usb_client(device)
         for index, channel in enumerate(device[CONF_CHANNELS]):
             chvar = cg.new_Pvariable(channel[CONF_ID], index, channel[CONF_BUFFER_SIZE])
             await cg.register_parented(chvar, var)
-            cg.add(chvar.set_rx_buffer_size(channel[CONF_BUFFER_SIZE]))
             cg.add(chvar.set_stop_bits(channel[CONF_STOP_BITS]))
             cg.add(chvar.set_data_bits(channel[CONF_DATA_BITS]))
             cg.add(chvar.set_parity(channel[CONF_PARITY]))
diff --git a/esphome/components/usb_uart/usb_uart.h b/esphome/components/usb_uart/usb_uart.h
index 8e8e65032d..f9648b795b 100644
--- a/esphome/components/usb_uart/usb_uart.h
+++ b/esphome/components/usb_uart/usb_uart.h
@@ -132,8 +132,9 @@ class USBUartChannel : public uart::UARTComponent, public Parented<USBUartCompon
   friend class USBUartTypeCH34X;
 
  public:
-  // Number of output chunk slots per channel (8 × 64 bytes = 512 bytes peak, lazily allocated)
-  static constexpr uint8_t USB_OUTPUT_CHUNK_COUNT = 8;
+  // Number of output chunk slots per channel, derived from buffer_size config.
+  // Computed as ceil(buffer_size / 64) + 1 in Python codegen; defaults to 5 (256 / 64 + 1).
+  static constexpr uint8_t USB_OUTPUT_CHUNK_COUNT = USB_UART_OUTPUT_CHUNK_COUNT;
 
   USBUartChannel(uint8_t index, uint16_t buffer_size) : index_(index), input_buffer_(RingBuffer(buffer_size)) {}
   void write_array(const uint8_t *data, size_t len) override;
diff --git a/esphome/core/defines.h b/esphome/core/defines.h
index d8b4faced9..1a8f3ef0bf 100644
--- a/esphome/core/defines.h
+++ b/esphome/core/defines.h
@@ -284,6 +284,7 @@
 #define ESPHOME_WIFI_POWER_SAVE_LISTENERS 2
 #define USE_WIFI_RUNTIME_POWER_SAVE
 #define USB_HOST_MAX_REQUESTS 16
+#define USB_UART_OUTPUT_CHUNK_COUNT 5
 
 #ifdef USE_ARDUINO
 #define USE_ARDUINO_VERSION_CODE VERSION_CODE(3, 3, 7)