4.12. Freescale i.MX

Freescale i.MX is traditionally very well supported under barebox. Depending on the SoC, there are different Boot Modes supported. Older SoCs up to i.MX31 support only the external Boot Mode. Newer SoCs can be configured for internal or external Boot Mode with the internal boot mode being the more popular mode. The i.MX23 and i.MX28, also known as i.MXs, are special. These SoCs have a completely different boot mechanism, see Freescale MXS instead.

4.12.1. Internal Boot Mode

The Internal Boot Mode is supported on:

  • i.MX25
  • i.MX35
  • i.MX50
  • i.MX51
  • i.MX53
  • i.MX6
  • i.MX7
  • i.MX8MQ

With the Internal Boot Mode, the images contain a header which describes where the binary shall be loaded and started. These headers also contain a so-called DCD table which consists of register/value pairs. These are executed by the Boot ROM and are used to configure the SDRAM. In barebox, the i.MX images are generated with the scripts/imx/imx-image tool. Normally it’s not necessary to call this tool manually, it is executed automatically at the end of the build process.

The images generated by the build process can be directly written to an SD card:

# with Multi Image support:
cat images/barebox-freescale-imx51-babbage.img > /dev/sdd
# otherwise:
cat barebox-flash-image > /dev/sdd

The above will overwrite the MBR (and consequently the partition table) on the destination SD card. To preserve the MBR while writing the rest of the image to the card, use:

dd if=images/barebox-freescale-imx51-babbage.img of=/dev/sdd bs=1024 skip=1 seek=1

Note that MaskROM on i.MX8 expects the image to start at the +33KiB mark, so the following command has to be used instead:

dd if=images/barebox-nxp-imx8mq-evk.img of=/dev/sdd bs=1024 skip=33 seek=33

Or, in case of NAND:

dd if=images/barebox-nxp-imx8mq-evk.img of=/dev/nand bs=1024 skip=33 seek=1

The images can also always be started as second stage on the target:

barebox@Board Name:/ bootm /mnt/tftp/barebox-freescale-imx51-babbage.img Using GPT on i.MX

For i.MX SoCs that place a vendor specific header at the +1KiB mark of a boot medium, special care needs to be taken when partitioning that medium with GPT. In order to make room for the i.MX boot header, the GPT Partition Entry Array needs to be moved from its typical location, LBA 2, to an offset past vendor specific information. One way to do this would be to use the -j or --adjust-main-table option of sgdisk. For example, the following sequence

sgdisk -Z <block device>
sgdisk -o -j 2048 -n 1:8192:+100M <block device>

will create a single GPT partition starting at LBA 8192 and would place the Partition Entry Array starting at LBA 2048, which should leave enough room for the Barebox/i.MX boot header. Once that is done, the dd command above can be used to place Barebox on the same medium. Information about the imx-image tool

The imx-image tool can be used to generate imximages from raw binaries. It requires an configuration file describing how to setup the SDRAM on a particular board. This mainly consists of a poke table. The recognized options in this file are:


soc <soctype>
soctype can be one of imx35, imx51, imx53, imx6, imx7, vf610,
loadaddr <adr> The address the binary is uploaded to
dcdofs <ofs>

The offset of the image header in the image. This should be:

  • 0x400: MMC/SD, NAND, serial ROM, PATA, SATA
  • 0x1000: NOR Flash
  • 0x100: OneNAND

Memory manipulation:

wm 8 <addr> <value> write <value> into byte <addr>
wm 16 <addr> <value> write <value> into short <addr>
wm 32 <addr> <value> write <value> into word <addr>
set_bits <width> <addr> <value> set set bits in <value> in <addr>
clear_bits <width> <addr> <value> clear set bits in <value> in <addr>
nop do nothing (just waste time)

<width> can be one of 8, 16 or 32.

Checking conditions:

check <width> <cond> <addr> <mask>

Poll until condition becomes true. with <cond> being one of:

  • until_all_bits_clear
  • until_all_bits_set
  • until_any_bit_clear
  • until_any_bit_set

Some notes about the mentioned conditions.

  • until_all_bits_clear waits until (*addr & mask) == 0 is true
  • until_all_bits_set waits until (*addr & mask) == mask is true
  • until_any_bit_clear waits until (*addr & mask) != mask is true
  • until_any_bit_set waits until (*addr & mask) != 0 is true. USB Boot

Most boards can be explicitly configured for USB Boot Mode or fall back to USB Boot when no other medium can be found. The barebox repository contains a USB upload tool. As it depends on the libusb development headers, it is not built by default. Enable it explicitly in make menuconfig and install the libusb development package. On Debian, this can be done with apt-get install libusb-dev. After compilation, the tool can be used with only the image name as argument:

scripts/imx/imx-usb-loader images/barebox-freescale-imx51-babbage.img

4.12.2. External Boot Mode

The External Boot Mode is supported by the older i.MX SoCs:

  • i.MX1
  • i.MX21
  • i.MX27
  • i.MX31
  • i.MX35

The External Boot Mode supports booting only from NOR and NAND flash. On NOR flash, the binary is started directly on its physical address in memory. Booting from NAND flash is more complicated. The NAND flash controller copies the first 2kb of the image to the NAND Controller’s internal SRAM. This initial binary portion then has to:

  • Set up the SDRAM
  • Copy the initial binary to SDRAM to make the internal SRAM in the NAND flash controller free for use for the controller
  • Copy the whole barebox image to SDRAM
  • Start the image

It is possible to write the image directly to NAND. However, since NAND flash can have bad blocks which must be skipped during writing the image and also by the initial loader, it is recommended to use the barebox_update - update barebox to persistent media command for writing to NAND flash.