Merge branch 'fix/log-dram-early-direct-rom-printf' into 'master'

fix(esp_log): Add Kconfig option to disable usage of esp_rom_vprintf when this function is in IRAM and not ROM

Closes IDFGH-17376

See merge request espressif/esp-idf!46640

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 for early/DRAM logs.
+
 Logging to Host via JTAG
 ------------------------