ota: detect Tasmota magic numbers when booting (#2370)

- inject dummy data sequence that is expected to hold current version info
- check rtcmem markers and purge settings, since we don't want accidentally reading something as a kv
- sometimes we cannot boot b/c of certain SDK params, purge last 16KiB as well

ref. `SetOption78 1`
https://tasmota.github.io/docs/Commands/#setoptions

The original idea from Esphome:
0e59243b83/esphome/core/esphal.cpp (L275-L287)
Does not seem to be working atm. Will need to re-check with the esphome builder, c/p did not work and I tried building with gcc-10

Specific things we want to do is to have 'version' byte between 2 'marker' bytes:
217addc2bb/tasmota/settings.ino (L218-L262)       0dfa38df89/tasmota/i18n.h (L780-L782)

code/espurna/main.cpp

@@ -153,15 +153,11 @@ void setup() {
     info(true);
 
     wifiSetup();
-    #if OTA_ARDUINOOTA_SUPPORT
-        arduinoOtaSetup();
-    #endif
+    otaSetup();
+
     #if TELNET_SUPPORT
         telnetSetup();
     #endif
-    #if OTA_CLIENT != OTA_CLIENT_NONE
-        otaClientSetup();
-    #endif
 
     // -------------------------------------------------------------------------
     // Check if system is stable

code/espurna/ota.cpp

@@ -7,8 +7,12 @@ OTA MODULE COMMON FUNCTIONS
 #include "ota.h"
 #include "system.h"
 #include "terminal.h"
+#include "rtcmem.h"
+#include "utils.h"
 #include "ws.h"
 
+#include <atomic>
+
 void otaPrintError() {
     if (Update.hasError()) {
         #if TERMINAL_SUPPORT
@@ -77,3 +81,45 @@ void otaProgress(size_t bytes, size_t each) {
         last = bytes;
     }
 }
+
+void otaSetup() {
+    // Some magic to allow seamless Tasmota OTA upgrades
+    // - inject dummy data sequence that is expected to hold current version info
+    // - purge settings, since we don't want accidentaly reading something as a kv
+    // - sometimes we cannot boot b/c of certain SDK params, purge last 16KiB
+    {
+        // ref. `SetOption78 1`
+        // - https://tasmota.github.io/docs/Commands/#setoptions (> SetOption78   Version check on Tasmota upgrade)
+        // - https://github.com/esphome/esphome/blob/0e59243b83913fc724d0229514a84b6ea14717cc/esphome/core/esphal.cpp#L275-L287 (the original idea from esphome)
+        // - https://github.com/arendst/Tasmota/blob/217addc2bb2cf46e7633c93e87954b245cb96556/tasmota/settings.ino#L218-L262 (specific checks, which succeed when finding 0xffffffff as version)
+        // - https://github.com/arendst/Tasmota/blob/0dfa38df89c8f2a1e582d53d79243881645be0b8/tasmota/i18n.h#L780-L782 (constants)
+        std::atomic_thread_fence(std::memory_order_relaxed);
+        volatile uint32_t magic[3] [[gnu::unused]] {
+            0x5AA55AA5,
+            0xFFFFFFFF,
+            0xA55AA55A
+        };
+
+        // ref. https://github.com/arendst/Tasmota/blob/217addc2bb2cf46e7633c93e87954b245cb96556/tasmota/settings.ino#L24
+        // We will certainly find these when rebooting from Tasmota. Purge SDK as well, since we may experience WDT after starting up the softAP
+        auto* rtcmem = reinterpret_cast<volatile uint32_t*>(RTCMEM_ADDR);
+        if ((0xA55A == rtcmem[64]) && (0xA55A == rtcmem[68])) {
+            DEBUG_MSG_P(PSTR("[OTA] Detected TASMOTA OTA, resetting the device...\n"));
+            rtcmem[64] = rtcmem[68] = 0;
+            customResetReason(CUSTOM_RESET_TERMINAL);
+            resetSettings();
+            eraseSDKConfig();
+            *((int*) 0) = 0;
+            // noreturn, we simply reboot after writing into 0
+        }
+
+        // TODO: also check for things throughout the flash sector, somehow?
+    }
+
+#if OTA_ARDUINOOTA_SUPPORT
+    arduinoOtaSetup();
+#endif
+#if OTA_CLIENT != OTA_CLIENT_NONE
+    otaClientSetup();
+#endif
+}

code/espurna/ota.h

@@ -38,6 +38,7 @@ void otaClientSetup();
 #include <WiFiClientSecure.h>
 #endif
 
+void otaSetup();
 void otaPrintError();
 bool otaFinalize(size_t size, int reason, bool evenIfRemaining = false);