ESP32 Partition size and maximum program size

Question

From an ESP32 example code, I extracted the following part, that just prints the flash partition list to the console.

#include <string.h>
#include <assert.h>
#include "esp_partition.h"
#include "esp_log.h"

static const char *TAG = "example";

void setup(void)
{
    Serial.begin(115200);
    delay(200); // Otherwise, the first log entries get lost
    ESP_LOGI(TAG, "Start test");
    esp_partition_iterator_t iter = esp_partition_find(ESP_PARTITION_TYPE_APP, ESP_PARTITION_SUBTYPE_ANY, NULL);
    ESP_LOGI(TAG, "Name, type, subtype, offset, length");
    while (iter != nullptr)
    {
      const esp_partition_t *partition = esp_partition_get(iter);
      ESP_LOGI(TAG, "%s, app, %d, 0x%x, 0x%x (%d)", partition->label, partition->subtype, partition->address, partition->size, partition->size);
      iter = esp_partition_next(iter);
    }
    
    esp_partition_iterator_release(iter);
    iter = esp_partition_find(ESP_PARTITION_TYPE_DATA, ESP_PARTITION_SUBTYPE_ANY, NULL);
    while (iter != nullptr)
    {
      const esp_partition_t *partition = esp_partition_get(iter);
      ESP_LOGI(TAG, "%s, data, %d, 0x%x, 0x%x (%d)", partition->label, partition->subtype, partition->address, partition->size, partition->size);
      iter = esp_partition_next(iter);
    }
    
    esp_partition_iterator_release(iter);

    
}

void loop()
{
}

In the project folder, I also have a custom partitions.csv file as follows:

nvs,        data, nvs,      0x9000,  0x6000,
phy_init,   data, phy,      0xf000,  0x1000,
factory,    app,  factory,  0x10000, 512k,
storage,    data, ,         ,        0x40000,

If I run the code, it correctly shows the custom partitions with the above sizes, everything like it should be.

However, the build output says the program uses 207278 Bytes from a maximum of 1310720. If I change the partition scheme selection in the tools menu, the second number changes. I would expect the maximum size to be equal to the size of the factory partition from the custom partition scheme.

If I change the factory partition to be only 100k in size, the ESP32 fails to boot and enters a core-dump-restart-loop.

Is this a compiler bug or am I overlooking something?

Answer 1

This appears to be a limitation of the toolchain. The output of processing the partitions.csv file doesn't make it back to the compiler to generate the correct message. The different partition schemes available in the menu come from boards.txt of the ESP32 package. They can be updated, but would be overwritten when the toolchain is updated. For the compiler input, the setting in the menu is relevant, but the actual size of the partitions comes from the partitions.csv file (if it exists).

This means that one has to manually make sure that the size of the factory partition in the csv file is larger or equal to the actual code size. Ideally, it is also equal to the selected partition size from the menu (to get the correct message and eventually error if the binary size exceeds the allocated space).

Some testing shows that if the actual size of the factory partition is smaller than the code size (and smaller than the compiler size limit) the application will successfully build and upload, but fail to run. The chip will enter a startup-reboot-forever loop and not execute any user code. However, whether that is always the case is not guaranteed.