Sub-projects like ULP are built as external CMake projects via
externalproject_add() and may use toolchains that are completely
different from the IDF target toolchain. For example, the ULP RISC-V
co-processor on an xtensa target (ESP32-S2/S3) uses
riscv32-esp-elf-gcc while the main project uses xtensa-esp-elf-gcc.
This causes problems in cmakev2:
1. __init_toolchain() in idf.cmake validates that CMAKE_TOOLCHAIN_FILE
matches IDF_TARGET and then overrides it. For sub-projects with
custom toolchains (passed via -DCMAKE_TOOLCHAIN_FILE from the
parent), this validation fails because the toolchain doesn't follow
the IDF naming convention (toolchain-<target>.cmake).
2. Several component project_include.cmake files use functions from
the IDF toolchain response file machinery (idf_toolchain_add_flags,
idf_toolchain_remove_flags, idf_toolchain_rerun_abi_detection) that
are not available in sub-projects with custom toolchains, or perform
validation that assumes the IDF target toolchain is in use.
Introduce IDF_CUSTOM_TOOLCHAIN, a CMake variable passed via -D from
the parent build to indicate that the toolchain was provided externally
and should not be resolved or validated by the IDF build system.
When IDF_CUSTOM_TOOLCHAIN is set:
- __init_toolchain() records the pre-set CMAKE_TOOLCHAIN_FILE and
skips IDF_TARGET-based resolution and validation.
- Component project_include.cmake files that manipulate IDF toolchain
flags (xtensa, esp_libc, esp_psram, soc) return early, as those
operations are not applicable to custom toolchains.
This is a generic mechanism usable by any sub-project that provides
its own toolchain (ULP, or any future sub-project with a non-IDF
toolchain).
Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>
This change improves build consistency across external projects integrated
through CMake by ensuring that compiler flags defined in configuration files
are passed correctly to the toolchain. It covers the majority of use cases,
as external projects are typically also CMake-based. For projects that use
a custom build system, users will still need to specify the required flags
manually.
