stand/altboot/README.altboot

/// notes about altboot ///

$NetBSD: README.altboot,v 1.2 2011/03/13 15:23:43 phx Exp $

Altboot is a functional bridge to fill the gap between a NAS product
custom bootloader and the NetBSD kernel startup environment.  Altboot
irons out and rectifies erroneously configured HW by product
bootloaders and prepares a sane runtime better suited for booting
NetBSD kernels.

- provides the foundation of a fast NetBSD porting cycle with functionalities
  product bootloaders don't have.
- facilitates a flexible and clean NetBSD implementation tailoured
  to target HW in detail, minimizing bumpy adjustments and hacks in
  locore asm and machdeps in very early kernel startup stage.
- levels out differences among similar-but-not-the-same porting
  targets to make it possible having common NetBSD kernels for them.
- builds and hands a bootinfo list to the NetBSD kernel.

Altboot is known working on at least three models.
- KuroBox with a popular U-Boot as the replacement of vendor proprietary

   U-Boot 1.1.4 LiSt 2.1.0 (Sep 21 2006 - 00:22:56) LinkStation / KuroBox

- Synology 101g+ with vendor custom PPCboot

   PPCBoot 2.0.0 (Mar  1 2005 - 15:31:41)

- D-Link DSM-G600 with heavily restricted vendor custom U-Boot

   U-Boot 0.2.0 (May 26 2005 - 19:38:32)

The standard use of altboot is to invoke it with a short script from
U-Boot/PPCboot, where the altboot image is stored in an unoccupied 128KB
section of the target's HW NOR flash.  Combined with standard
U-Boot/PPCboot functions, it is possible to boot a NetBSD kernel off
it right after power-on, without the help of manual intervention.  Note
that the original U-Boot/PPCboot still remains useful and altboot works
as a functional extension for them.

Altboot hands the following bootinfo records to the NetBSD/sandpoint
kernel.
- processor clock tick value driving MPC8241/8245.
- serial console selection.
- booted kernel filename and which device it was fetched from.
- Ethernet MAC address, if target HW lacks SEEPROM to store a unit unique
  value.
- product family indication.
- preloaded kernel module names (under development).

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