4.9. 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.

4.9.1. Internal Boot Mode

The Internal Boot Mode is supported on:

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

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=512 skip=1 seek=1

The images can also always be started second stage:

bootm /mnt/tftp/barebox-freescale-imx51-babbage.img

4.9.2. Internal Boot Mode Through Internal RAM(IRAM)

The Internal Boot Mode Through Internal RAM is supported on:

  • i.MX51

As can be easily deduced from its name, the Internal Boot Mode Through Internal RAM is just a variant of Internal Boot Mode so all of the stated above still applies in this case. What it differs in is the following:

  • Boot process is done in two stages(First stage binary can be produced with imx_v7-xload_defconfig)
  • DCD of the first stage image is set such that the image is fetched into an unoccupied area or IRAM
  • First stage image once uncompressed and set up will look for a second stage bootloader on the same media it booted from and start it(see mach-imx/xload.c for more details)
  • Second stage images are just regular i.MX boot images

Since on a typical i.MX SoC unused IRAM area is not enough to run anything but a PBL this mode, due to its very limited usability, serves only one purpose – allow for a portion of a bootloader to be executed without depending on DRAM to be functional. This peculiarity of the mode can be used to implement various memory testing scenarious.

4.9.2.1. 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

4.9.3. 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.