From 877ba13f4f36624ca37ff2263be7d34b4b62def2 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Mon, 15 Sep 2025 17:56:58 -0500
Subject: [PATCH] [ethernet] Conditionally compile PHY-specific code to reduce
 flash usage

---
 esphome/components/ethernet/__init__.py            | 11 +++++++++++
 esphome/components/ethernet/ethernet_component.cpp |  4 ++++
 esphome/components/ethernet/ethernet_component.h   |  2 ++
 esphome/core/defines.h                             |  1 +
 4 files changed, 18 insertions(+)

diff --git a/esphome/components/ethernet/__init__.py b/esphome/components/ethernet/__init__.py
index a26238553c..151da7d0e5 100644
--- a/esphome/components/ethernet/__init__.py
+++ b/esphome/components/ethernet/__init__.py
@@ -77,6 +77,13 @@ ETHERNET_TYPES = {
     "DM9051": EthernetType.ETHERNET_TYPE_DM9051,
 }
 
+# PHY types that need compile-time defines for conditional compilation
+_PHY_TYPE_TO_DEFINE = {
+    "KSZ8081": "USE_ETHERNET_KSZ8081",
+    "KSZ8081RNA": "USE_ETHERNET_KSZ8081",
+    # Add other PHY types here only if they need conditional compilation
+}
+
 SPI_ETHERNET_TYPES = ["W5500", "DM9051"]
 SPI_ETHERNET_DEFAULT_POLLING_INTERVAL = TimePeriodMilliseconds(milliseconds=10)
 
@@ -345,6 +352,10 @@ async def to_code(config):
     if CONF_MANUAL_IP in config:
         cg.add(var.set_manual_ip(manual_ip(config[CONF_MANUAL_IP])))
 
+    # Add compile-time define for PHY types with specific code
+    if phy_define := _PHY_TYPE_TO_DEFINE.get(config[CONF_TYPE]):
+        cg.add_define(phy_define)
+
     cg.add_define("USE_ETHERNET")
 
     # Disable WiFi when using Ethernet to save memory
diff --git a/esphome/components/ethernet/ethernet_component.cpp b/esphome/components/ethernet/ethernet_component.cpp
index a48fd27383..ff14d19427 100644
--- a/esphome/components/ethernet/ethernet_component.cpp
+++ b/esphome/components/ethernet/ethernet_component.cpp
@@ -229,10 +229,12 @@ void EthernetComponent::setup() {
   ESPHL_ERROR_CHECK(err, "ETH driver install error");
 
 #ifndef USE_ETHERNET_SPI
+#ifdef USE_ETHERNET_KSZ8081
   if (this->type_ == ETHERNET_TYPE_KSZ8081RNA && this->clk_mode_ == EMAC_CLK_OUT) {
     // KSZ8081RNA default is incorrect. It expects a 25MHz clock instead of the 50MHz we provide.
     this->ksz8081_set_clock_reference_(mac);
   }
+#endif  // USE_ETHERNET_KSZ8081
 
   for (const auto &phy_register : this->phy_registers_) {
     this->write_phy_register_(mac, phy_register);
@@ -721,6 +723,7 @@ bool EthernetComponent::powerdown() {
 
 #ifndef USE_ETHERNET_SPI
 
+#ifdef USE_ETHERNET_KSZ8081
 constexpr uint8_t KSZ80XX_PC2R_REG_ADDR = 0x1F;
 
 void EthernetComponent::ksz8081_set_clock_reference_(esp_eth_mac_t *mac) {
@@ -749,6 +752,7 @@ void EthernetComponent::ksz8081_set_clock_reference_(esp_eth_mac_t *mac) {
     ESP_LOGVV(TAG, "KSZ8081 PHY Control 2: %s", format_hex_pretty((u_int8_t *) &phy_control_2, 2).c_str());
   }
 }
+#endif  // USE_ETHERNET_KSZ8081
 
 void EthernetComponent::write_phy_register_(esp_eth_mac_t *mac, PHYRegister register_data) {
   esp_err_t err;
diff --git a/esphome/components/ethernet/ethernet_component.h b/esphome/components/ethernet/ethernet_component.h
index 3d2713ee5c..bbb9d7fb60 100644
--- a/esphome/components/ethernet/ethernet_component.h
+++ b/esphome/components/ethernet/ethernet_component.h
@@ -104,8 +104,10 @@ class EthernetComponent : public Component {
   void start_connect_();
   void finish_connect_();
   void dump_connect_params_();
+#ifdef USE_ETHERNET_KSZ8081
   /// @brief Set `RMII Reference Clock Select` bit for KSZ8081.
   void ksz8081_set_clock_reference_(esp_eth_mac_t *mac);
+#endif
   /// @brief Set arbitratry PHY registers from config.
   void write_phy_register_(esp_eth_mac_t *mac, PHYRegister register_data);
 
diff --git a/esphome/core/defines.h b/esphome/core/defines.h
index 9a7e090b83..6e8d5ed74c 100644
--- a/esphome/core/defines.h
+++ b/esphome/core/defines.h
@@ -175,6 +175,7 @@
 #ifdef USE_ARDUINO
 #define USE_ARDUINO_VERSION_CODE VERSION_CODE(3, 2, 1)
 #define USE_ETHERNET
+#define USE_ETHERNET_KSZ8081
 #endif
 
 #ifdef USE_ESP_IDF