From cd350825c72a43078d556a49d3cf2920df5b2d5e Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Wed, 19 Aug 2026 14:14:28 -0500
Subject: [PATCH] Only re-wake the loop once the RX buffer is drained, reset
 the frame state in decoder setup, test reconfiguration

---
 .../uart/software_serial_rx_decoder.h         |  4 ++
 .../uart/uart_component_esp8266.cpp           |  6 ++-
 .../uart/test_software_serial_rx_decoder.cpp  | 53 +++++++++++++++++++
 3 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/esphome/components/uart/software_serial_rx_decoder.h b/esphome/components/uart/software_serial_rx_decoder.h
index 29c31b3c36..6c09258183 100644
--- a/esphome/components/uart/software_serial_rx_decoder.h
+++ b/esphome/components/uart/software_serial_rx_decoder.h
@@ -22,6 +22,8 @@ class SoftwareSerialRxDecoder {
  public:
   static constexpr uint8_t RX_IDLE = 0xFF;
 
+  /// Configure the framing and buffer. Drops buffered bytes and any partial frame and
+  /// assumes an idle high line; call reset() afterwards with the real line level.
   void setup(uint32_t bit_cycles, uint8_t data_bits, bool parity, uint8_t stop_bits, uint8_t *buffer,
              size_t buffer_size) {
     this->bit_cycles_ = bit_cycles;
@@ -33,11 +35,13 @@ class SoftwareSerialRxDecoder {
     this->buffer_size_ = buffer_size;
     this->in_pos_ = 0;
     this->out_pos_ = 0;
+    this->reset(0, true);
   }
 
   /// Forget any partial frame; `level` is the current line level.
   void reset(uint32_t now, bool level) {
     this->bit_ = RX_IDLE;
+    this->cur_byte_ = 0;
     this->last_level_ = level;
     this->last_edge_ = now;
   }
diff --git a/esphome/components/uart/uart_component_esp8266.cpp b/esphome/components/uart/uart_component_esp8266.cpp
index 0eab780114..514510a6cf 100644
--- a/esphome/components/uart/uart_component_esp8266.cpp
+++ b/esphome/components/uart/uart_component_esp8266.cpp
@@ -315,8 +315,10 @@ void IRAM_ATTR ESP8266SoftwareSerial::gpio_intr_edge(ESP8266SoftwareSerial *arg)
 void ESP8266SoftwareSerial::rx_finalize_pending_() {
   if (!this->rx_.finalize_due(arch_get_cpu_cycle_count())) {
 #ifdef USE_UART_WAKE_LOOP_ON_RX
-    // Not old enough yet: run the loop again right away instead of after a full loop_interval_.
-    wake_loop_threadsafe();
+    // Not old enough yet: once the caller has drained what is there, run the loop
+    // again right away instead of after a full loop_interval_.
+    if (this->rx_.available() == 0)
+      wake_loop_threadsafe();
 #endif
     return;
   }
diff --git a/tests/components/uart/test_software_serial_rx_decoder.cpp b/tests/components/uart/test_software_serial_rx_decoder.cpp
index c8961fc3ad..89e65f759f 100644
--- a/tests/components/uart/test_software_serial_rx_decoder.cpp
+++ b/tests/components/uart/test_software_serial_rx_decoder.cpp
@@ -219,6 +219,59 @@ TEST(SoftwareSerialRxDecoder, DropsBytesWhenBufferIsFullAndKeepsOldest) {
     EXPECT_EQ(sim.received()[n], n);
 }
 
+TEST(SoftwareSerialRxDecoder, SetupAgainDropsStaleStateAndUsesNewBufferAndFraming) {
+  // Mirrors load_settings(): bytes buffered and a frame left open under 8N1 in a
+  // 64 byte buffer, then setup() again with 5E2 in a 4 byte buffer.
+  LineSim sim(9600, 8, false, false, 1);
+  uint32_t t = 5000;
+  for (int n = 0; n < 10; n++)
+    t = sim.send(static_cast<uint8_t>(0x40 + n), t);
+  sim.edge(t + 8 * sim.bit_cycles(), false);  // open a frame, never closed
+  SoftwareSerialRxDecoder &dec = sim.decoder();
+  ASSERT_GE(dec.available(), 9u);
+
+  std::vector<uint8_t> small(4, 0xEE);
+  const uint32_t bit = CPU_HZ / 2400;
+  dec.setup(bit, 5, true, 2, small.data(), small.size());
+  EXPECT_EQ(dec.available(), 0u);
+  EXPECT_FALSE(dec.pending());
+  EXPECT_EQ(dec.read_byte(), 0);
+
+  // 5E2 frames fed straight into the reconfigured decoder: capacity is 3, the
+  // rest are dropped and nothing is written past the end of the new buffer.
+  auto send_5e2 = [&](uint8_t value, uint32_t start) {
+    bool line = true;
+    uint32_t at = start;
+    auto put = [&](bool b) {
+      if (b != line) {
+        dec.on_edge(at, b);
+        line = b;
+      }
+      at += bit;
+    };
+    put(false);
+    int ones = 0;
+    for (int i = 0; i < 5; i++) {
+      bool b = (value >> i) & 1;
+      ones += b;
+      put(b);
+    }
+    put(ones & 1);
+    put(true);
+    put(true);
+    return at;
+  };
+  uint32_t t2 = 5000;
+  for (int n = 1; n <= 6; n++)
+    t2 = send_5e2(static_cast<uint8_t>(n), t2);
+  dec.finalize(t2 + 20 * bit);
+  ASSERT_EQ(dec.available(), 3u);
+  EXPECT_EQ(dec.read_byte(), 1);
+  EXPECT_EQ(dec.read_byte(), 2);
+  EXPECT_EQ(dec.read_byte(), 3);
+  EXPECT_EQ(small[3], 0xEE);  // capacity slot is never written
+}
+
 TEST(SoftwareSerialRxDecoder, ResetDiscardsPartialFrame) {
   LineSim sim(9600, 8, false, false, 1);
   const uint32_t bit = sim.bit_cycles();