fix(esp_log): add Kconfig to optionally bypass esp_rom_vprintf

In Log V2, ESP_LOG macros are using the runtime
constrained-environment path in esp_log_vprintf() reference
esp_rom_vprintf as the fallback formatter. On chips where ets_vprintf
is not in ROM (all targets except ESP32-S2 and ESP32-C2), this pulls
~1.2 KB of printf implementation into IRAM.

Add a new Kconfig option LOG_API_CONSTRAINED_ENV_SAFE (default y) that
controls whether this IRAM-resident fallback is used:

Also update the logging and RAM-usage documentation accordingly.

Closes https://github.com/espressif/esp-idf/issues/18346

components/log/Kconfig

@@ -52,6 +52,30 @@ menu "Log"
 
     rsource "./Kconfig.settings"
 
+    config LOG_API_CONSTRAINED_ENV_SAFE
+        bool "Enable safe ESP_LOGx fallback in constrained environments (ISR, cache disabled)"
+        depends on LOG_VERSION_2 && !ESP_ROM_HAS_VPRINTF_FUNC && !LOG_MODE_BINARY_EN
+        default y
+        help
+            This option is only relevant on chips where ets_vprintf is NOT in ROM
+
+            When enabled, Log V2 routes ESP_DRAM_LOGx, ESP_EARLY_LOGx, and runtime
+            constrained-environment log calls (ISR, cache disabled) through esp_log(),
+            which uses esp_rom_vprintf for formatting.
+
+            When disabled, ESP_DRAM_LOGx and ESP_EARLY_LOGx expand directly
+            to esp_rom_printf (a true ROM function with zero IRAM cost), bypassing
+            esp_log(). Normal ESP_LOGx calls that happen to run in a constrained
+            environment (ISR, cache disabled) will use the standard vprintf function
+            instead of esp_rom_vprintf; if the standard vprintf resides in flash,
+            such calls may crash. Use ESP_DRAM_LOGx for any logging that must work
+            with cache disabled or from an ISR.
+
+            Disable this option to save ~1.2 KB of IRAM on memory-constrained devices.
+            Enable it if you need ESP_LOGx to safely fall back to ROM-based printing
+            in ISRs or with cache disabled, or if you need constrained-environment
+            logs in binary log format.
+
     config LOG_IN_IRAM
         bool "Place logging functions in IRAM" if SPI_FLASH_AUTO_SUSPEND
         default y

components/log/include/esp_log.h

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2015-2025 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2015-2026 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -247,14 +247,14 @@ void esp_log_va(esp_log_config_t config, const char *tag, const char *format, va
 
 #if defined(__cplusplus) && (__cplusplus >  201703L)
 #define ESP_LOG_EARLY_IMPL(tag, format, configs, log_tag_letter, ...) do { \
-    if (ESP_LOG_VERSION == 1) { \
+    if (ESP_LOG_VERSION == 1 || (ESP_LOG_VERSION == 2 && !ESP_LOG_API_CONSTRAINED_ENV_SAFE)) { \
         if (_ESP_LOG_EARLY_ENABLED(configs)) { esp_rom_printf(LOG_FORMAT(log_tag_letter, format), esp_log_timestamp(), tag __VA_OPT__(,) __VA_ARGS__); } \
     } else { \
         if (ESP_LOG_ENABLED(configs)) { esp_log(ESP_LOG_CONFIG_INIT((configs) | ESP_LOG_CONFIGS_DEFAULT | ESP_LOG_CONFIG_CONSTRAINED_ENV), tag, ESP_LOG_ATTR_STR(format) ESP_LOG_ARGS(__VA_ARGS__)); } \
     } } while(0)
 #else // !(defined(__cplusplus) && (__cplusplus >  201703L))
 #define ESP_LOG_EARLY_IMPL(tag, format, configs, log_tag_letter, ...) do { \
-    if (ESP_LOG_VERSION == 1) { \
+    if (ESP_LOG_VERSION == 1 || (ESP_LOG_VERSION == 2 && !ESP_LOG_API_CONSTRAINED_ENV_SAFE)) { \
         if (_ESP_LOG_EARLY_ENABLED(configs)) { esp_rom_printf(LOG_FORMAT(log_tag_letter, format), esp_log_timestamp(), tag, ##__VA_ARGS__); } \
     } else { \
         if (ESP_LOG_ENABLED(configs)) { esp_log(ESP_LOG_CONFIG_INIT((configs) | ESP_LOG_CONFIGS_DEFAULT | ESP_LOG_CONFIG_CONSTRAINED_ENV), tag, ESP_LOG_ATTR_STR(format) ESP_LOG_ARGS(__VA_ARGS__)); } \
@@ -263,14 +263,14 @@ void esp_log_va(esp_log_config_t config, const char *tag, const char *format, va
 
 #if defined(__cplusplus) && (__cplusplus >  201703L)
 #define ESP_DRAM_LOG_IMPL(tag, format, configs, log_tag_letter, ...) do { \
-    if (ESP_LOG_VERSION == 1) { \
+    if (ESP_LOG_VERSION == 1 || (ESP_LOG_VERSION == 2 && !ESP_LOG_API_CONSTRAINED_ENV_SAFE)) { \
         if (_ESP_LOG_EARLY_ENABLED(configs)) { esp_rom_printf(_ESP_LOG_DRAM_LOG_FORMAT(log_tag_letter, format), tag __VA_OPT__(,) __VA_ARGS__); } \
     } else { \
         if (ESP_LOG_ENABLED(configs)) { esp_log(ESP_LOG_CONFIG_INIT((configs) | ESP_LOG_CONFIGS_DEFAULT | ESP_LOG_CONFIG_CONSTRAINED_ENV | ESP_LOG_CONFIG_DIS_COLOR | ESP_LOG_CONFIG_DIS_TIMESTAMP), tag, ESP_LOG_ATTR_DRAM_STR(format) ESP_LOG_ARGS(__VA_ARGS__)); } \
     } } while(0)
 #else // !(defined(__cplusplus) && (__cplusplus >  201703L))
 #define ESP_DRAM_LOG_IMPL(tag, format, configs, log_tag_letter, ...)  do { \
-    if (ESP_LOG_VERSION == 1) { \
+    if (ESP_LOG_VERSION == 1 || (ESP_LOG_VERSION == 2 && !ESP_LOG_API_CONSTRAINED_ENV_SAFE)) { \
         if (_ESP_LOG_EARLY_ENABLED(configs)) { esp_rom_printf(_ESP_LOG_DRAM_LOG_FORMAT(log_tag_letter, format), tag, ##__VA_ARGS__); } \
     } else { \
         if (ESP_LOG_ENABLED(configs)) { esp_log(ESP_LOG_CONFIG_INIT((configs) | ESP_LOG_CONFIGS_DEFAULT | ESP_LOG_CONFIG_CONSTRAINED_ENV | ESP_LOG_CONFIG_DIS_COLOR | ESP_LOG_CONFIG_DIS_TIMESTAMP), tag, ESP_LOG_ATTR_DRAM_STR(format) ESP_LOG_ARGS(__VA_ARGS__)); } \

components/log/include/esp_log_config.h

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2024-2025 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2024-2026 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -13,6 +13,7 @@
 #include "esp_log_level.h"
 #include "esp_assert.h"
 #include "sdkconfig.h"
+#include "esp_rom_caps.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -111,6 +112,25 @@ extern "C" {
 #ifndef ESP_LOG_FORMATTING_DISABLED
 #define ESP_LOG_FORMATTING_DISABLED              (0)
 #endif
+
+/**
+ * This define controls whether esp_rom_vprintf is used as the fallback print function in constrained
+ * environments (ISR, cache disabled, early startup). When set to 0, esp_rom_vprintf is never referenced,
+ * saving ~1.2 KB of IRAM on chips where it is not a true ROM function.
+ * ESP_DRAM_LOGx and ESP_EARLY_LOGx will use esp_rom_printf directly instead.
+ *
+ * On chips where ets_vprintf is in ROM (ESP32-S2, ESP32-C2), this is always enabled
+ * since there is no IRAM cost.
+ */
+#if BOOTLOADER_BUILD || ESP_ROM_HAS_VPRINTF_FUNC
+#define ESP_LOG_API_CONSTRAINED_ENV_SAFE  (1)
+#else
+#ifdef CONFIG_LOG_API_CONSTRAINED_ENV_SAFE
+#define ESP_LOG_API_CONSTRAINED_ENV_SAFE  (1)
+#else
+#define ESP_LOG_API_CONSTRAINED_ENV_SAFE  (0)
+#endif
+#endif
 /** @endcond */
 
 /**

components/log/include/esp_private/log_print.h

@@ -32,11 +32,15 @@ __attribute__((always_inline)) static inline void esp_log_vprintf(esp_log_config
 #else // APP
     extern vprintf_like_t esp_log_vprint_func;
 #if ESP_LOG_VERSION == 2
+#if CONFIG_LOG_API_CONSTRAINED_ENV_SAFE
     vprintf_like_t vprint_func[2] = {
         esp_log_vprint_func,
         esp_rom_vprintf,
     };
     vprint_func[config.opts.constrained_env](format, args);
+#else
+    esp_log_vprint_func(format, args);
+#endif // CONFIG_LOG_API_CONSTRAINED_ENV_SAFE
 #else // ESP_LOG_VERSION == 1
     esp_log_vprint_func(format, args);
 #endif // ESP_LOG_VERSION == 1

docs/en/api-guides/performance/ram-usage.rst

@@ -194,6 +194,7 @@ The following options will reduce IRAM usage of some ESP-IDF features:
     - Disable :ref:`CONFIG_LIBC_LOCKS_PLACE_IN_IRAM` if no ISRs that run while cache is disabled (i.e. IRAM ISRs) use libc lock APIs.
     :CONFIG_ESP_ROM_HAS_SUBOPTIMAL_NEWLIB_ON_MISALIGNED_MEMORY: - Disable :ref:`CONFIG_LIBC_OPTIMIZED_MISALIGNED_ACCESS` to save approximately 1000 bytes of IRAM, at the cost of reduced performance.
     :SOC_SPIRAM_SUPPORTED: - Enable :ref:`CONFIG_ESP_EVENT_LOOP_IN_EXT_RAM` to force ``esp_event`` to place event loop related allocations in external RAM instead of internal RAM.
+    :not CONFIG_ESP_ROM_HAS_VPRINTF_FUNC: - When using **Log V2**, disable :ref:`CONFIG_LOG_API_CONSTRAINED_ENV_SAFE` to remove ``esp_rom_vprintf`` from IRAM, saving ~1.2 KB. This means ``ESP_LOGx`` will no longer safely fall back to ROM-based printing in ISRs or with cache disabled. Use ``ESP_DRAM_LOGx`` explicitly for constrained-environment logging. See :doc:`/api-reference/system/log` for details.
 
 .. only:: esp32
 

docs/en/api-reference/system/log.rst

@@ -656,6 +656,23 @@ The following measurements were performed using the ``esp_timer`` example with d
 
 Enabling **Log V2** increases IRAM usage while reducing the overall application binary size, Flash code, and data usage.
 
+.. only:: not CONFIG_ESP_ROM_HAS_VPRINTF_FUNC
+
+    Reducing IRAM Usage in Log V2
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+    The IRAM increase in **Log V2** is primarily caused by ``esp_rom_vprintf``, which is compiled into IRAM (~1.2 KB) on {IDF_TARGET_NAME}. This function is referenced as the fallback formatter for constrained environments (ISR, cache disabled) in the ``esp_log_vprintf()`` inline function.
+
+    On chips where IRAM and DRAM share the same memory pool, this also reduces available heap by the same amount.
+
+    To eliminate this cost, disable :ref:`CONFIG_LOG_API_CONSTRAINED_ENV_SAFE` (enabled by default). When disabled:
+
+    - ``ESP_DRAM_LOGx`` and ``ESP_EARLY_LOGx`` expand directly to ``esp_rom_printf()`` (a true ROM function, zero IRAM cost), bypassing the ``esp_log()`` pipeline entirely.
+    - Normal ``ESP_LOGx`` calls in constrained environments (ISR, cache disabled) will use the standard ``vprintf`` function. If ``vprintf`` resides in flash, such calls may crash. Use ``ESP_DRAM_LOGx`` for any logging that must work with cache disabled or from an ISR.
+    - ``esp_rom_vprintf`` is never referenced, so the linker excludes it from the binary.
+
+    When enabled, the original **Log V2** behavior is preserved: all constrained-environment logs route through ``esp_log()`` and use ``esp_rom_vprintf`` as the formatter, supporting features like binary log format for early/DRAM logs.
+
 Logging to Host via JTAG
 ------------------------