From 662f14e10ba7355054aed80c8b7eb0a9075d7126 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Thu, 12 Mar 2026 12:16:16 -1000
Subject: [PATCH] [debug] Implement reset reason for RP2040/RP2350

The debug component's reset reason text sensor was returning an empty
string on RP2040/RP2350 platforms. Read the chip reset registers to
report the actual reset source.

RP2040: reads VREG_AND_CHIP_RESET for POR, RUN pin, and debug port.
RP2350: reads POWMAN chip_reset for POR, brown-out, RUN pin, watchdog
variants, glitch detect, debugger, rescue, and core powerdown.
Both: checks watchdog_caused_reboot() from the Pico SDK.

Also adds buf_append_str() helper to helpers.h for efficient plain
string appends without format parsing overhead.
---
 esphome/components/debug/debug_rp2040.cpp | 65 ++++++++++++++++++++++-
 esphome/core/helpers.h                    | 22 ++++++++
 2 files changed, 86 insertions(+), 1 deletion(-)

diff --git a/esphome/components/debug/debug_rp2040.cpp b/esphome/components/debug/debug_rp2040.cpp
index c9d41942dbc..24757b8810e 100644
--- a/esphome/components/debug/debug_rp2040.cpp
+++ b/esphome/components/debug/debug_rp2040.cpp
@@ -2,12 +2,75 @@
 #ifdef USE_RP2040
 #include "esphome/core/log.h"
 #include <Arduino.h>
+#include <hardware/watchdog.h>
+#if defined(PICO_RP2350)
+#include <hardware/structs/powman.h>
+#else
+#include <hardware/structs/vreg_and_chip_reset.h>
+#endif
 namespace esphome {
 namespace debug {
 
 static const char *const TAG = "debug";
 
-const char *DebugComponent::get_reset_reason_(std::span<char, RESET_REASON_BUFFER_SIZE> buffer) { return ""; }
+const char *DebugComponent::get_reset_reason_(std::span<char, RESET_REASON_BUFFER_SIZE> buffer) {
+  char *buf = buffer.data();
+  const size_t size = RESET_REASON_BUFFER_SIZE;
+  size_t pos = 0;
+
+#if defined(PICO_RP2350)
+  uint32_t chip_reset = powman_hw->chip_reset;
+  if (chip_reset & 0x10000000)  // HAD_WATCHDOG_RESET_RSM
+    pos = buf_append_str(buf, size, pos, "Watchdog (RSM)|");
+  if (chip_reset & 0x08000000)  // HAD_HZD_SYS_RESET_REQ
+    pos = buf_append_str(buf, size, pos, "Hazard debugger reset|");
+  if (chip_reset & 0x04000000)  // HAD_GLITCH_DETECT
+    pos = buf_append_str(buf, size, pos, "Power supply glitch|");
+  if (chip_reset & 0x02000000)  // HAD_SWCORE_PD
+    pos = buf_append_str(buf, size, pos, "Switched core powerdown|");
+  if (chip_reset & 0x01000000)  // HAD_WATCHDOG_RESET_SWCORE
+    pos = buf_append_str(buf, size, pos, "Watchdog (SWCORE)|");
+  if (chip_reset & 0x00800000)  // HAD_WATCHDOG_RESET_POWMAN
+    pos = buf_append_str(buf, size, pos, "Watchdog (POWMAN)|");
+  if (chip_reset & 0x00400000)  // HAD_WATCHDOG_RESET_POWMAN_ASYNC
+    pos = buf_append_str(buf, size, pos, "Watchdog (POWMAN async)|");
+  if (chip_reset & 0x00200000)  // HAD_RESCUE
+    pos = buf_append_str(buf, size, pos, "Rescue reset|");
+  if (chip_reset & 0x00080000)  // HAD_DP_RESET_REQ
+    pos = buf_append_str(buf, size, pos, "Debugger reset|");
+  if (chip_reset & 0x00040000)  // HAD_RUN_LOW
+    pos = buf_append_str(buf, size, pos, "RUN pin|");
+  if (chip_reset & 0x00020000)  // HAD_BOR
+    pos = buf_append_str(buf, size, pos, "Brown-out|");
+  if (chip_reset & 0x00010000)  // HAD_POR
+    pos = buf_append_str(buf, size, pos, "Power-on reset|");
+#else
+  uint32_t chip_reset = vreg_and_chip_reset_hw->chip_reset;
+  if (chip_reset & 0x00100000)  // HAD_PSM_RESTART
+    pos = buf_append_str(buf, size, pos, "Debug port restart|");
+  if (chip_reset & 0x00010000)  // HAD_RUN
+    pos = buf_append_str(buf, size, pos, "RUN pin|");
+  if (chip_reset & 0x00000100)  // HAD_POR
+    pos = buf_append_str(buf, size, pos, "Power-on reset|");
+#endif
+
+  if (watchdog_caused_reboot()) {
+    if (watchdog_enable_caused_reboot()) {
+      pos = buf_append_str(buf, size, pos, "Watchdog timeout|");
+    } else {
+      pos = buf_append_str(buf, size, pos, "Watchdog reboot|");
+    }
+  }
+
+  // Remove trailing '|'
+  if (pos > 0 && buf[pos - 1] == '|') {
+    buf[pos - 1] = '\0';
+  } else if (pos == 0) {
+    return "Unknown";
+  }
+
+  return buf;
+}
 
 const char *DebugComponent::get_wakeup_cause_(std::span<char, RESET_REASON_BUFFER_SIZE> buffer) { return ""; }
 
diff --git a/esphome/core/helpers.h b/esphome/core/helpers.h
index 70ac1574f0c..d28d53be023 100644
--- a/esphome/core/helpers.h
+++ b/esphome/core/helpers.h
@@ -942,6 +942,28 @@ __attribute__((format(printf, 4, 5))) inline size_t buf_append_printf(char *buf,
 }
 #endif
 
+/// Safely append a string to buffer without format parsing, returning new position (capped at size).
+/// More efficient than buf_append_printf for plain string literals.
+/// @param buf Output buffer
+/// @param size Total buffer size
+/// @param pos Current position in buffer
+/// @param str String to append
+/// @return New position after appending (capped at size on overflow)
+inline size_t buf_append_str(char *buf, size_t size, size_t pos, const char *str) {
+  if (pos >= size) {
+    return size;
+  }
+  size_t remaining = size - pos - 1;  // reserve space for null terminator
+  size_t len = strlen(str);
+  if (len > remaining) {
+    len = remaining;
+  }
+  memcpy(buf + pos, str, len);
+  pos += len;
+  buf[pos] = '\0';
+  return pos;
+}
+
 /// Concatenate a name with a separator and suffix using an efficient stack-based approach.
 /// This avoids multiple heap allocations during string construction.
 /// Maximum name length supported is 120 characters for friendly names.