.. SPDX-License-Identifier: GPL-2.0+

Nokia RX-51 aka N900
====================

The ``nokia_rx51_defconfig`` configuration file results in a ``u-boot.bin``
which can be chainloaded by the Nokia second stage bootloader (NOLO) in QEMU or
on a real N900. It does very little hardware configuration because NOLO has
already configured the board. It is only needed to enable the internal eMMC
memory via the twl4030 regulator which is not enabled by NOLO.

NOLO is expecting a kernel image and will treat any image it finds in
OneNAND as such. This u-boot is intended to be flashed to the N900 like
a kernel. In order to transparently boot the original kernel, it will be
appended to ``u-boot.bin`` at 0x40000. NOLO will load the entire image into
(random) memory and execute U-Boot, which saves hardware revision, boot reason
and boot mode ATAGs set by NOLO. Then the bootscripts will attempt to load
``uImage``, ``zImage`` or ``boot.scr`` file from a FAT or ext2/3/4 filesystem
on external SD card or internal eMMC memory. If this fails or keyboard is
closed then the appended kernel image will be booted using some generated
and some stored ATAGs (see boot order).

For generating combined image of U-Boot and kernel (either in uImage or zImage
format) there is a simple script called ``u-boot-gen-combined``. It is available
in following repository: https://github.com/pali/u-boot-maemo.

To generate the ``combined.bin`` image from ``u-boot.bin`` and ``kernel.bin``
(in either uImage or zImage format) use:

.. code-block:: bash

   sh u-boot-gen-combined u-boot.bin kernel.bin combined.bin

The original Maemo Fremantle PR1.3 zImage kernel binary is available at:
http://repository.maemo.org/pool/maemo5.0/free/k/kernel/kernel_2.6.28-20103103+0m5_armel.deb

To unpack it (from DEB/AR, TAR and FIASCO) execute the following commands:

.. code-block:: bash

   ar x kernel_2.6.28-20103103+0m5_armel.deb data.tar.gz
   tar -O -xf data.tar.gz ./boot/zImage-2.6.28-20103103+0m5.fiasco > kernel_2.6.28-20103103+0m5.fiasco
   0xFFFF -M kernel_2.6.28-20103103+0m5.fiasco -u

The flashed image must start with a 2 KiB ``NOLO!img`` header which contains
size of the image. The header consists of the bytes
``NOLO!img\x02\x00\x00\x00\x00\x00\x00\x00`` followed by the 4 byte little
endian size of the image. The rest of the 2 KiB header just contains zero bytes.

The Nokia proprietary flasher and also the open source 0xFFFF flasher
automatically prepend the required ``NOLO!img`` header and both applications
expect that the image does not contain a ``NOLO!img`` header. Adding a
``NOLO!img`` header is required only in case of using the ``nandwrite`` tool for
flashing.

The open source 0xFFFF flasher is available in the following repository:
https://github.com/pali/0xFFFF

It is possible to load ``u-boot.bin`` via USB to the N900 RAM and boot it
without needing to flash it. This is done via 0xFFFF running on the host PC:

.. code-block:: bash

   0xFFFF -m u-boot.bin -l -b

0xFFFF also supports flashing a kernel image either via USB or directly on
N900 device. Flashing u-boot/kernel/combined image is done as:

.. code-block:: bash

   0xFFFF -m combined.bin -f

Via 0xFFFF it is also possible to generate a standard flashable image in
Nokia FIASCO format which contains metadata information like device
identification (RX-51) and version string (v2021.04):

.. code-block:: bash

   0xFFFF -m RX-51:v2021.04:kernel:u-boot.bin -g u-boot.fiasco

There is support for the hardware watchdog. The hardware watchdog is started by
NOLO so U-Boot must reset the watchdog to prevent rebooting the device (but not
very often, max every 2 seconds). There is also support for framebuffer display
output with ANSI escape codes and the N900 hardware keyboard input.

When U-Boot is starting it sets the IBE bit in the Auxiliary Control Register,
which is needed for Thumb-2 ISA support. This is a workaround for erratum
430973.

Default boot order
------------------

0. if keyboard is closed boot automatically attached kernel image
1. try boot from external SD card
2. try boot from internal eMMC memory
3. try boot from attached kernel image

Boot from SD or eMMC in this order:

1. boot from FAT partition

    a. find ``boot.scr`` on first FAT partition
    b. find ``uImage`` on first FAT partition
    c. find ``zImage`` on first FAT partition
    d. same order for 2nd - 4th FAT partition

2. same as 1. but for ext2/3/4 partition

Available additional commands/variables
---------------------------------------

* run sdboot - Boot from external SD card (see boot order)
* run emmcboot - Boot from internal eMMC memory (see boot order)
* run attachboot - Boot attached kernel image (attached to U-Boot binary)

\

* run scriptload - Load boot script ``${mmcscriptfile}``
* run scriptboot - Run loaded boot script
* run kernload - Load kernel image ``${mmckernfile}``
* run initrdload - Load initrd image ``${mmcinitrdfile}``
* run kernboot - Boot loaded kernel image
* run kerninitrdboot - Boot loaded kernel image with loaded initrd image

\

* run trymmcscriptboot - Try to load and boot script ``${mmcscriptfile}``
* run trymmckernboot - Try to load and boot kernel image ``${mmckernfile}``
* run trymmckerninitrdboot - Try to load and boot kernel image ``${mmckernfile}``
  with initrd image ``${mmcinitrdfile}``

Additional variables for loading files from mmc
-----------------------------------------------

* mmc ``${mmcnum}`` (0 - external, 1 - internal)
* partition number ``${mmcpart}`` (1 - 4)
* partition type ``${mmctype}`` (fat, ext2, ext4; ext2 is just alias for ext4)

Additional variables for booting a kernel
-----------------------------------------

* ``setup_omap_atag`` - Add OMAP table into atags structure (needed for maemo kernel)
* ``setup_console_atag`` - Enable serial console in OMAP table
* ``setup_boot_reason_atag`` - Change boot reason in OMAP table
* ``setup_boot_mode_atag`` - Change boot mode in OMAP table

Variable ``setup_omap_atag`` is automatically set when booting attached kernel.
When variable ``setup_omap_atag`` is set, variable ``setup_console_atag`` is unset
and u-boot standard output is set to serial then ``setup_console_atag`` is
automatically set to 1. So output from Maemo kernel would go to serial port.

UBIFS support
-------------

UBIFS support is disabled, because U-Boot image is too big and cannot be
flashed with attached kernel image to RX-51 kernel nand area. For enabling
UBIFS support add following lines into file ``configs/nokia_rx51_defconfig``::

    CONFIG_CMD_UBI=y
    CONFIG_CMD_UBIFS=y
    CONFIG_MTD_UBI_FASTMAP=y
    CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT=1
