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Documentation/esp32s3-devkit: Describe the kernel build configurations.

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Describe kernel_oct and kernel_n8r2 next to the other configurations of this
board.

The entry for kernel_oct carries what a user needs and cannot guess:  a KERNEL
build is the only mode with fork() on this chip, the page pool is reached
through a scratch mapping rather than a permanent window, the ROMFS is linked
into the kernel image so a change to an application needs the whole
export-import-mkromfsimg-relink chain, how to confirm that the ROMFS is really
in the image, and that the shell needs the full path of a program.

The entry for kernel_n8r2 states its limit.  Each region of a process is 2
pages, ostest has 115 KiB of text against a 128 KiB text region, and a larger
program needs a module with more PSRAM.

Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
This commit is contained in:
Marco Casaroli 2026-08-10 22:01:57 +02:00 • committed by Alan C. Assis
parent 1a061d9b4e
commit bfb2cd5541
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Documentation/platforms/xtensa/esp32s3/boards/esp32s3-devkit/index.rst
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@ -372,6 +372,63 @@ After successfully built and flashed, run on the boards's terminal::
The corresponding output should show related debug information.
kernel_n8r2
-----------
A KERNEL build for a module with 2 MB of PSRAM, such as the
ESP32-S3-WROOM-1-N8R2. The flash is quad and runs in DIO mode.
The page pool is smaller than in ``kernel_oct``, and each of the text, data
and heap regions of a process is 2 pages of 64 KiB. A process therefore takes
384 KiB, and a ``fork()`` needs 768 KiB while the parent and the child both
exist.
This is tight on purpose. ``ostest`` has 115 KiB of text against a 128 KiB
text region. A larger program needs a module with more PSRAM, not a larger
page pool.
kernel_oct
----------
A KERNEL build for a module with octal flash and 8 MB of PSRAM, such as the
ESP32-S3-WROOM-2-N32R8V.
A KERNEL build gives each process its own address environment. This is the
only build mode in which ``fork()`` is available on this chip: the child
receives its own copy of the memory of the parent, at the same virtual
addresses. ``vfork()`` is available in every build mode.
The page pool holds the pages of every process. It is carved out of the
PSRAM, and it is not kept mapped into the kernel address space. The kernel
reaches a page of the pool through a small scratch region, mapped for one
operation and invalidated afterwards.
Build the kernel first, then the applications, then the boot ROMFS, and then
link the kernel again. The ROMFS is linked into the kernel image, so a change
to an application needs the whole chain::
make -j
make export
cd ../apps
./tools/mkimport.sh -z -x ../nuttx/nuttx-export-*.tar.gz
make import -j
./tools/mkromfsimg.sh ../nuttx/arch/xtensa/src/board/board/romfs_boot.c
cd ../nuttx
make -j
Confirm that the ROMFS is in the image. ``romfs_img`` must be a strong symbol
of a few hundred kilobytes::
xtensa-esp32s3-elf-nm -S nuttx | grep " romfs_img$"
A one byte symbol means that the linker took the stub. Delete
``boards/xtensa/esp32s3/esp32s3-devkit/src/romfs_stub.o`` and ``libboard.a``,
then link again.
The shell needs the full path of a program in this build mode::
nsh> /system/bin/ostest
knsh
----