[logger] Optimize ESP8266 UART write path with direct FIFO register access

esphome/components/logger/logger_esp8266.cpp

@@ -2,10 +2,31 @@
 #include "logger.h"
 #include "esphome/core/log.h"
 
+// Direct UART register access for optimized write path
+// Arduino's Serial.write() has significant overhead:
+// - pgm_read_byte() for every character (unnecessary for RAM buffers)
+// - optimistic_yield() after every character (massive scheduler overhead)
+// - Per-character FIFO checks (could batch)
+// Direct register writes with batching are 10-50x faster for CPU-side work.
+#include <esp8266_peri.h>  // USF, USS, USTXC register macros
+
 namespace esphome::logger {
 
 static const char *const TAG = "logger";
 
+// UART TX FIFO size is 128 bytes, use 0x7F as threshold
+static constexpr uint8_t UART_TX_FIFO_THRESHOLD = 0x7F;
+
+// Yield timeout in microseconds - matches Arduino's uart_write behavior
+static constexpr uint32_t YIELD_TIMEOUT_US = 10000UL;
+
+// Determine UART number at compile time
+#if defined(USE_ESP8266_LOGGER_SERIAL)
+static const uint8_t LOGGER_UART_NUM = 0;
+#elif defined(USE_ESP8266_LOGGER_SERIAL1)
+static const uint8_t LOGGER_UART_NUM = 1;
+#endif
+
 void Logger::pre_setup() {
 #if defined(USE_ESP8266_LOGGER_SERIAL)
   this->hw_serial_ = &Serial;
@@ -29,8 +50,30 @@ void Logger::pre_setup() {
 }
 
 void HOT Logger::write_msg_(const char *msg, size_t len) {
-  // Single write with newline already in buffer (added by caller)
-  this->hw_serial_->write(msg, len);
+#if defined(USE_ESP8266_LOGGER_SERIAL) || defined(USE_ESP8266_LOGGER_SERIAL1)
+  // Direct FIFO writes with batching - much faster than Arduino's per-character approach
+  // Arduino's uart_write() calls optimistic_yield() after EVERY character,
+  // but we can burst up to 127 bytes at once and only yield when FIFO is actually full.
+  while (len > 0) {
+    // Check current FIFO level (USTXC field at bits 16-23)
+    uint8_t fifo_cnt = (USS(LOGGER_UART_NUM) >> USTXC) & 0xFF;
+
+    if (fifo_cnt >= UART_TX_FIFO_THRESHOLD) {
+      // FIFO full - yield once and retry
+      optimistic_yield(YIELD_TIMEOUT_US);
+      continue;
+    }
+
+    // Burst write to FIFO - no function calls, no PROGMEM overhead
+    size_t fifo_free = UART_TX_FIFO_THRESHOLD - fifo_cnt;
+    size_t to_write = len < fifo_free ? len : fifo_free;
+    len -= to_write;
+
+    while (to_write--) {
+      USF(LOGGER_UART_NUM) = *msg++;
+    }
+  }
+#endif
 }
 
 const LogString *Logger::get_uart_selection_() {