bugfix(uart_building): Fix IRAM usage when building uart for IDF 5.5

ESP-IDF v5.5 fails to build for UART transport on co-processor due to
not enough IRAM space

- reduce IRAM use by moving ringbuffer to flash
- added note to UART doc about this
- cause a build error if UART is build without config to reduce IRAM
  usage

Author: SohKamYung-Espressif

docs/uart.md

@@ -146,6 +146,15 @@ Configure the co-processor project using
 idf.py menuconfig
 ```
 
+> [!NOTE]
+> When building with ESP-IDF v5.5, you may see this build error:
+>
+> `Building for UART transport can fail due to lack of IRAM space`
+>
+> To reduce IRAM usage, you can either:
+> - run `idf.py menuconfig` and enable `Component config` --> `ESP Ringbuf` ---> `Place non-ISR ringbuf functions into flash`, or
+> - uncomment `CONFIG_RINGBUF_PLACE_FUNCTIONS_INTO_FLASH=y` in `slave/sdkconfig.defaults.esp32` and regenerate the `sdkconfig`
+
 #### 6.2.1 Transport config
   - Navigate to "Example configuration" -> "Bus Config in between Host and Co-processor"
   - In "Transport layer", select "UART"

slave/main/uart_slave_api.c

@@ -1,17 +1,8 @@
-// SPDX-License-Identifier: Apache-2.0
-// Copyright 2015-2024 Espressif Systems (Shanghai) PTE LTD
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
+/*
+ * SPDX-FileCopyrightText: 2015-2025 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
 
 #include "sdkconfig.h"
 
@@ -28,6 +19,7 @@
 #include "interface.h"
 #include "mempool.h"
 #include "stats.h"
+#include "esp_idf_version.h"
 #include "esp_hosted_interface.h"
 #include "esp_hosted_transport.h"
 #include "esp_hosted_transport_init.h"
@@ -47,6 +39,20 @@
 
 #define BUFFER_SIZE                MAX_TRANSPORT_BUF_SIZE
 
+#if (ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(5, 0, 0)) && (ESP_IDF_VERSION < ESP_IDF_VERSION_VAL(6, 0, 0))
+/**
+ * For ESP-IDF v5.5, Building with UART Transport can fail due to
+ * lack of IRAM space.
+ * To reduce IRAM usage, `CONFIG_RINGBUF_PLACE_FUNCTIONS_INTO_FLASH=y`
+ * should be enabled
+ */
+#if !CONFIG_RINGBUF_PLACE_FUNCTIONS_INTO_FLASH
+#error Building for UART transport can fail due to lack of IRAM space
+#error To free up IRAM, enable Component config --> ESP Ringbuf ---> Place non-ISR ringbuf functions into flash
+#error or uncomment CONFIG_RINGBUF_PLACE_FUNCTIONS_INTO_FLASH=y in sdkconfig.defaults.esp32 and regenerate sdkconfig
+#endif
+#endif
+
 static const char TAG[] = "UART_DRIVER";
 
 // UART is low throughput, so throttling should not be needed

slave/sdkconfig.ci.uart

@@ -1 +1,4 @@
+### To reduce IRAM usage on ESP-IDF v5.5
+CONFIG_RINGBUF_PLACE_FUNCTIONS_INTO_FLASH=y
+
 CONFIG_ESP_UART_HOST_INTERFACE=y

slave/sdkconfig.defaults.esp32

@@ -37,6 +37,10 @@ CONFIG_PARTITION_TABLE_CUSTOM_FILENAME="partitions.esp32.csv"
 # iram text saving
 CONFIG_FREERTOS_PLACE_FUNCTIONS_INTO_FLASH=y
 
+# for ESP-IDF v5.5, enable this setting if building for UART transport
+# needed to reduce iram usage
+# CONFIG_RINGBUF_PLACE_FUNCTIONS_INTO_FLASH=y
+
 # OS
 CONFIG_BOOTLOADER_COMPILER_OPTIMIZATION_SIZE=y
 CONFIG_BOOTLOADER_COMPILER_OPTIMIZATION_PERF=n