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docs: secure boot v2: Add a note about maximum bootloader size

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Angus Gratton 2020-03-20 15:40:01 +11:00 committed by Angus Gratton
parent cf8dd62fc4
commit 7c34711c07
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docs/en/security/secure-boot-v2.rst
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@ -106,15 +106,24 @@ An image is “verified” if the public key stored in any signature block is va
Bootloader Size
---------------
{IDF_TARGET_MAX_BOOTLOADER_SIZE:default = "64KB (0x10000 bytes)", esp32 = "48KB (0xC000 bytes)"}
{IDF_TARGET_MAX_PARTITION_TABLE_OFFSET:default = "0x12000", esp32 = "0xE000"}
.. Above is calculated as 0x1000 at start of flash + IDF_TARGET_MAX_BOOTLOADER_SIZE + 0x1000 signature sector
When secure boot is enabled the bootloader app binary ``bootloader.bin`` may exceed the default bootloader size limit. This is especially likely if flash encryption is enabled as well. The default size limit is 0x7000 (28672) bytes (partition table offset 0x8000 - bootloader offset 0x1000).
If the bootloader becomes too large, the ESP32 will fail to boot - errors will be logged about either invalid partition table or invalid bootloader checksum.
If the bootloader becomes too large, the {IDF_TARGET_NAME} will fail to boot - errors will be logged about either invalid partition table or invalid bootloader checksum.
When Secure Boot V2 is enabled, there is also an absolute binary size limit of {IDF_TARGET_MAX_BOOTLOADER_SIZE} (excluding the 4KB signature), because the bootloader is first loaded into a fixed size buffer for verification.
Options to work around this are:
- Set :ref:`bootloader compiler optimization <CONFIG_BOOTLOADER_COMPILER_OPTIMIZATION>` back to "Size" if it has been changed from this default value.
- Reduce :ref:`bootloader log level <CONFIG_BOOTLOADER_LOG_LEVEL>`. Setting log level to Warning, Error or None all significantly reduce the final binary size (but may make it harder to debug).
- Set :ref:`partition table offset <CONFIG_PARTITION_TABLE_OFFSET>` to a higher value than 0x8000, to place the partition table later in the flash. This increases the space available for the bootloader. If the :doc:`partition table </api-guides/partition-tables>` CSV file contains explicit partition offsets, they will need changing so no partition has an offset lower than ``CONFIG_PARTITION_TABLE_OFFSET + 0x1000``. (This includes the default partition CSV files supplied with ESP-IDF.)
Note that because of the absolute binary size limit, there is no benefit to moving the partition table any higher than offset {IDF_TARGET_MAX_PARTITION_TABLE_OFFSET}.
.. _efuse-usage:
eFuse usage