ARM bootloader installation in Guix

[Danny Milosavljevic]

Hi,

I've been thinking of ways to generalize the bootloader installation for
ARM boards.  One of the most annoying parts of u-boot is that the u-boot project
does NOT include bootloader installers (something like grub-install does not
exist in u-boot).

A possible upstream project that builds ARM images is Buildroot (it's basically
a Linux distribution).

First, it would [probably] be possible to use Buildroot directly, but they have
different policies like using *specific* u-boot versions, toolchain and compiler
versions etc--also it takes a LOT of time and space to do that, and for 
something
as tiny as u-boot that would be kinda silly.

A second way would be to use the buildroot config files and "genimage.cfg"s
in order to extract (1) which u-boot configuration to use and (2) have genimage
build the bootloader parts of the image (or the entire image just with empty 
rootfs).

For that, we would (for example):

* Read configs/orangepi_zero_defconfig to find:
  BR2_TARGET_UBOOT=y
  BR2_TARGET_UBOOT_BOARD_DEFCONFIG="orangepi_zero"
  BR2_TARGET_UBOOT_NEEDS_DTC=y
  BR2_TARGET_UBOOT_FORMAT_CUSTOM=y
  BR2_TARGET_UBOOT_FORMAT_CUSTOM_NAME="u-boot-sunxi-with-spl.bin"
  BR2_TARGET_UBOOT_BOOT_SCRIPT=y
  BR2_TARGET_UBOOT_BOOT_SCRIPT_SOURCE="board/orangepi/orangepi-zero/boot.cmd"
  BR2_PACKAGE_HOST_DOSFSTOOLS=y
  BR2_PACKAGE_HOST_GENIMAGE=y
  BR2_PACKAGE_HOST_MTOOLS=y
  BR2_PACKAGE_HOST_UBOOT_TOOLS=y
  BR2_ROOTFS_POST_IMAGE_SCRIPT="support/scripts/genimage.sh"
  BR2_ROOTFS_POST_SCRIPT_ARGS="-c board/orangepi/orangepi-zero/genimage.cfg"

* Then read board/orangepi/orangepi-zero/genimage.cfg to find:

  image boot.vfat { ... which files to put there }
  image sdcard.img { ... which files and partitions to put there }

We could reimplement genimage ourselves, but I suggest we just use genimage
instead (I've packaged it in guix master).

The rootfs size would need to be dynamically adjusted (the "genimage.cfg"s
usually specify a fixed size)--not sure how annoying that would be.

That way we could generalize u-boot bootloader installation without undue
maintenance effort on our side.

A possible way forward could be to implement an importer to figure out all
the boards to put into gnu/bootloader/u-boot.scm, including an installer that
uses genimage in order to install the bootloader (and, really, ideally
partitions everything else--so it should run much earlier or else we'd have to
do extra modifications that could be avoided).

I've started branch "wip-buildroot" in order to do that.

To try it out:

* Invoke: guix import buildroot sheevaplug_defconfig
* Stick the result into gnu/bootloader/u-boot.scm (with appropriate "define")
* Build system image using it.

(It's not finished yet but it should provide a basis for discussion...)

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