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Installing Linux Images to Flash Print


This note explains how to install a bootable Linux image to the NOR Flash on the STM32F4 System-On-Module (SOM). Having installed a Linux image to the Flash, you would be able to boot Linux on the target automatically on any power up or reset cycle. This is the mode of operation that is typically used for deployed production units.

It should be noted that all STM32F4 SOMs come with a default Linux image preloaded to the Flash. This is the same rootfs.uImage that is included in the Emcraft software distribution (refer to STM32F4 System-On-Module Release Materials). Clearly, as you proceed to develop your Linux applications against your specific requirements, you will need a way to install an updated Linux image to the target for deployment. This note explains how to do that.

The example below refers to rootfs.uImage. This image is included as a prebuilt binary in the software distribution so you can always use it to revert to the factory default software, should you need to do that at some point. As mentioned above though, the process below will work for any bootable Linux image you build using the Emcraft software distribution.

Before you proceed to install a Linux image to the NOR Flash, it probably makes sense to verify that what you have is indeed a bootable Linux image. You can do that by loading the image to the target from the network using the run netboot command (refer to Loading Linux Images via Ethernet and TFTP). It is important to understand that the same uImage that you load via TFTP can be loaded from the Flash as well. It doesn't make sense to install a Linux image to the Flash unless you have validated the image by loading it via Ethernet and TFTP.

The U-Boot command sequence required to install a Linux image to the NOR Flash is defined for you by the U-Boot environment variable called update. Here is how update is defined in U-Boot by default:

STM32F4X9-SOM> print update
update=tftp ${image};prot off ${flashaddr} +${filesize};era ${flashaddr} +${filesize};cp.b ${loadaddr} ${flashaddr} ${filesize};setenv kernelsize ${filesize};setenv filesize; setenv fileaddr;saveenv
STM32F4X9-SOM>

You will notice that the first command in update is a tftpboot command to download an image from the network via Ethernet and TFTP. Please refer to Loading Linux Images via Ethernet and TFTP for detailed information on how to set up your target and the development host for loading images from TFTP. Following tftpboot, there is a sequence of commands that erase those Flash sections where the Linux image will be installed to and then copy the image to the Flash.

Here is a snapshot showing how update is used to install rootfs.uImage to the Flash:

STM32F4X9-SOM> setenv image vlad/rootfs.uImage
STM32F4X9-SOM> saveenv 
STM32F4X9-SOM> run update
Auto-negotiation...completed.
STM32_MAC: link UP (100/Half)
Using STM32_MAC device
TFTP from server 172.17.0.1; our IP address is 172.17.4.206
Filename 'vlad/rootfs.uImage'.
Load address: 0xc0007fb4
Loading: #################################################################
#################################################################
#################################################################
#################################################################
#################################################################
##
done
Bytes transferred = 4794223 (49276f hex)
..................................... done
Un-Protected 37 sectors

..................................... done
Erased 37 sectors
Copy to Flash... done
Saving Environment to Flash...
. done
Un-Protected 1 sectors
Erasing Flash...
. done
Erased 1 sectors
Writing to Flash... done
. done
Protected 1 sectors
STM32F4X9-SOM>

Having installed an image to the Flash, just hit the Reset button on the baseboard to boot the newly installed image from Flash:

U-Boot 2010.03-cortexm-2.0.0 (May 5 2016 - 12:31:28)

CPU  : STM32F4 (Cortex-M4)
Freqs: SYSCLK=180MHz,HCLK=180MHz,PCLK1=45MHz,PCLK2=90MHz
Board: STM-SOM Rev 2.A, www.emcraft.com
DRAM:  32 MB
Flash: 16 MB
I...
## Booting kernel from Legacy Image at c0007fb4 ...
Image Name:   Linux-4.2.0-00175-g21f52b5-dirty
Image Type:   ARM Linux Multi-File Image (uncompressed)
Data Size:    4794159 Bytes =  4.6 MB
Load Address: c0008000
Entry Point:  c0008001
Contents:
Image 0: 4776496 Bytes =  4.6 MB
Image 1: 17651 Bytes = 17.2 kB
Verifying Checksum ... OK
## Flattened Device Tree from multi component Image at C0007FB4
Booting using the fdt at 0xc0496230
Loading Multi-File Image ... OK
OK
Loading Device Tree to c1ff8000, end c1fff4f2 ... OK

Starting kernel ...

Booting Linux on physical CPU 0x0
Linux version 4.2.0-00175-g21f52b5-dirty (vlad @ocean.emcraft.com) (gcc version 4.4.1 (Sourcery G++ Lite 2010q1-189) ) #26 Sun Mar 20 15:35:11 MSK 2016
...
init started: BusyBox v1.17.0 (2016-03-09 16:27:43 MSK)
...
/ #

On deployed units, you will probably want to optimize the boot time by setting the U-Boot bootdelay variable to 0. Please refer to Autobooting Linux from U-Boot for detailed information on how to do that.