xref: /openbmc/u-boot/board/gateworks/gw_ventana/README (revision 40e999ca)

U-Boot for the Gateworks Ventana Product Family boards

This file contains information for the port of U-Boot to the Gateworks
Ventana Product family boards.

The entire Ventana product family (http://www.gateworks.com/product#ventana)
is supported by a single bootloader build by using a common SPL and U-Boot
that dynamically determines the characterstics of the board at runtime via
information from an EEPROM on the board programmed at the factory and supports
all of the various boot mediums available.

1. Secondary Program Loader (SPL)
---------------------------------

The i.MX6 has a BOOT ROM PPL (Primary Program Loader) which supports loading
an executable image from various boot devices.

The Gateworks Ventana board config uses an SPL build configuration. This
will build the following artifacts from u-boot source:
 - SPL - Secondary Program Loader that the i.MX6 BOOT ROM (Primary Program
         Loader) boots.  This detects CPU/DRAM configuration, configures
         The DRAM controller, loads u-boot.img from the detected boot device,
         and jumps to it.  As this is booted from the PPL, it has an IVT/DCD
         table.
 - u-boot.img - The main u-boot core which is u-boot.bin with a image header.


2. Build
--------

To build U-Boot for the Gateworks Ventana product family:

 make gwventana_config
 make


3. Boot source:
---------------

The Gateworks Ventana boards support booting from NAND or micro-SD depending
on the board model. The IMX6 BOOT ROM will choose a boot media based on eFUSE
settings programmed at the factory.

Boards with NAND flash will always boot from NAND, and NAND-less boards will
always boot from micro-SD. However, it is possible to use the U-Boot bmode
command (or the technique it uses) to essentially bootstrap to another boot
media at runtime.

3.1. boot from NAND
-------------------

The i.MX6 BOOT ROM expects some structures that provide details of NAND layout
and bad block information (referred to as 'bootstreams') which are replicated
multiple times in NAND. The number of replications and their spacing (referred
to as search stride) is configurable through board strapping options and/or
eFUSE settings (BOOT_SEARCH_COUNT / Pages in block from BOOT_CFG2). In
addition, the i.MX6 BOOT ROM Flash Configuration Block (FCB) supports two
copies of a bootloader in flash in the case that a bad block has corrupted one.
The Freescale 'kobs-ng' application from the Freescale LTIB BSP, which runs
under Linux and operates on an MTD partition, must be used to program the
bootstream in order to setup this flash structure correctly.

The Gateworks Ventana boards with NAND flash have been factory programmed
such that their eFUSE settings expect 2 copies of the boostream (this is
specified by providing kobs-ng with the --search_exponent=1 argument). Once in
Linux with MTD support for the NAND on /dev/mtd0 you can program the SPL
with:

kobs-ng init -v -x --search_exponent=1 SPL

The kobs-ng application uses an imximage which contains the Image Vector Table
(IVT) and Device Configuration Data (DCD) structures that the i.MX6 BOOT ROM
requires to boot.  The kobs-ng adds the Firmware Configuration Block (FCB) and
Discovered Bad Block Table (DBBT).  The SPL build artifact from u-boot is
an imximage.

The u-boot.img, which is the non SPL u-boot binary appended to a u-boot image
header must be programmed in the NAND flash boot device at an offset hard
coded in the SPL. For the Ventana boards, this has been chosen to be 14MB.
The image can be programmed from either u-boot or Linux:

u-boot:
Ventana > setenv mtdparts mtdparts=nand:14m(spl),2m(uboot),1m(env),-(rootfs)
Ventana > tftp ${loadaddr} u-boot.img && nand erase.part uboot && \
          nand write ${loadaddr} uboot ${filesize}

Linux:
nandwrite /dev/mtd1 u-boot.img

The above assumes the default Ventana partitioning scheme which is configured
via the mtdparts env var:
 - spl: 14MB
 - uboot: 2M
 - env: 1M
 - rootfs: the rest

This information is taken from:
  http://trac.gateworks.com/wiki/ventana/bootloader#nand

More details about the i.MX6 BOOT ROM can be found in the IMX6 reference manual.

3.1. boot from micro-SD
-----------------------

When the IMX6 eFUSE settings have been factory programmed to boot from
micro-SD the SPL will be loaded from offset 0x400 (1KB). Once the SPL is
booted, it will load and execute U-boot (u-boot.img) from offset 69KB
on the micro-SD (defined by CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR).

While it is technically possible to enable the SPL to be able to load
U-Boot from a file on a FAT/EXT filesystem on the micro-SD, we chose to
use raw micro-SD access to keep the code-size and boot time of the SPL down.

For these reasons a micro-SD that will be used as an IMX6 primary boot
device must be carefully partitioned and prepared.

The following shell commands are executed on a Linux host (adjust DEV to the
block storage device of your micro-SD):

 DEV=/dev/sdc
 # zero out 1MB of device
 sudo dd if=/dev/zero of=$DEV count=1 bs=1M oflag=sync status=none && sync
 # copy SPL to 1KB offset
 sudo dd if=SPL of=$DEV bs=1K seek=1 oflag=sync status=none && sync
 # copy U-Boot to 69KB offset
 sudo dd if=u-boot.img of=$DEV bs=1K seek=69 oflag=sync status=none && sync
 # create a partition table with a single rootfs partition starting at 1MB
 printf "1,,L\n" | sudo sfdisk --in-order --no-reread -L -uM $DEV && sync
 # format partition
 sudo mkfs.ext4 -L root ${DEV}1
 # mount the partition
 sudo udisks --mount ${DEV}1
 # extract filesystem
 sudo tar xvf rootfs.tar.gz -C /media/root
 # flush and unmount
 sync && sudo umount /media/root

The above assumes the default Ventana micro-SD partitioning scheme
 - spl    :   1KB-69KB  (68KB)  required by IMX6 BOOT ROM
 - uboot  :  69KB-709KB (640KB) defined by
                                CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR
 - env    : 709KB-965KB (256KB) defined by
                                CONFIG_ENV_MMC_SIZE
                                CONFIG_ENV_MMC_OFFSET_REDUND
 - rootfs :   1MB-

This information is taken from:
  http://trac.gateworks.com/wiki/ventana/bootloader#microsd

More details about the i.MX6 BOOT ROM can be found in the IMX6 reference manual.