4.15. Freescale MXS

Freescale i.MXs or MXS are a SoC family which consists of the i.MX23 and the i.MX28. These are quite different from the regular i.MX SoCs and thus are represented by their own architecture in both the kernel and in barebox.

4.15.1. Bootlets

Traditionally these SoCs need the Freescale bootlets source and the elf2sb2 binary to set up the PMIC and SDRAM, and to build a bootable image out of the barebox binary. Since the bootlets are board-specific and the source code is only hardly customisable, each vendor usually has their own slightly different version of the bootlets. Booting with the Freescale bootlets is not described here, refer to the bootlet source code or your vendor’s documentation instead.

Barebox has a port of the bootlets integrated into its source, which is derived from the U-Boot bootlet code written by Marek Vasut.

Most MXS boards in the barebox tree have been ported to use barebox bootlets and image generation, and we recommend this approach for new boards too.

4.15.2. Booting Freescale i.MXs

The Freescale MXS SoCs have a multi staged boot process which needs different images composed out of different binaries. The ROM executes a so called bootstream which can contain multiple executables.

The first executable (the prepare stage) is only a small binary executed in SRAM, which has a limited size of 128 KB. Its purpose is to setup the internal PMIC and the SDRAM, and then jump back to the MXS ROM code, which then maps the second executable (the full bootloader) into SDRAM and executes it. In case of barebox, the bootstream (*-bootstream.img) is composed of the self extracting barebox image (*.pblx) and the prepare stage (prep_*.pblb). The file name of those images reflects the name of the respective entry points.

The bootstream image itself is useful for USB boot, but for booting from SD cards or NAND a BCB header has to be prepended to the image. In case of SD boot the image is named *-sd.img.

Bootstream images can be unencrypted or encrypted. Depending on the OCOTP fuses of your chip, you might need the one or the other to boot the board.

Since some of the bootstream images are encrypted, they are not suitable for 2nd stage execution. For this purpose a 2nd stage image with the name *-2nd.img is generated.

4.15.3. Booting from USB

If enabled in menuconfig → System Type, barebox builds the imx-usb-loader tool (derived from the sbloader tool from the rockbox project), which can load images onto MXS SoCs over USB. (Refer to the documentation of your board how to get it into USB boot mode.)

If the board is connected to the PC and started in USB boot mode, it should show up in lsusb:

Bus 001 Device 098: ID 15a2:004f Freescale Semiconductor, Inc. i.MX28 SystemOnChip in RecoveryMode

The bootstream images can then directly be booted with:

./scripts/imx-usb-loader images/barebox-karo-tx28-bootstream.img

You might require appropriate udev rules or sudo to gain the rights to access the USB device.

4.15.4. Booting from SD cards

The SD images are suitable for booting from SD cards. The MXS SoCs require a special partition of type 0x53 (OnTrack DM6 Aux) which contains the BCB header. The partitioning can be created with the following fdisk sequence (using /dev/sdg as an example for the SD card):

$ fdisk /dev/sdg

Welcome to fdisk (util-linux 2.25.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.


Command (m for help): o
Created a new DOS disklabel with disk identifier 0xd7e5d328.

Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-7829503, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-7829503, default 7829503): +1M

Created a new partition 1 of type 'Linux' and of size 1 MiB.

Command (m for help): t
Selected partition 1
Hex code (type L to list all codes): 53
Changed type of partition 'Linux' to 'OnTrack DM6 Aux3'.

Command (m for help): w

After writing the new partition table, the image can be written directly to the first partition:

cat images/barebox-karo-tx28-sd.img > /dev/sdg1

Note

For some unknown reason the BCB header is inside a partition, but contains the sector number of the raw device from which the rest of the image is read. With standard settings, booting from SD card only works if the partition containing the bootloader starts at sector 2048 (the standard for fdisk). See the -p parameter to the scripts/mxsboot tool to change this sector number in the image.

4.15.5. Booting second stage

The second stage images can be started with the barebox bootm - boot an application image command or just jumped into using the go - start application at address or file command.