Support for this syntax is still present in current versions, but will be removed soon. In the current implementation, the local variables space and global environment variables space are separated. Note that local variables can only be used for simple commands, not for compound commands etc. Global environment variables are those you can set and print using setenv and printenv.

Author:Migul Jur
Language:English (Spanish)
Published (Last):2 October 2018
PDF File Size:10.68 Mb
ePub File Size:6.27 Mb
Price:Free* [*Free Regsitration Required]

The last output shows the partition to be empty. When we tried to write the new date again, we got an error. The reason is that the date has changed probably at least the seconds and flash memory cannot be simply overwritten - it has to be erased first. The mkfs. After all, a flash device is not a normal harddisk. Access times to the files on the flash filesystem may increase drastically. Attempts to truncate a file to free space or to rename it may fail This is especially critical when you are using the flash filesystem to store log files: when your application detects some abnormal condition and produces lots of log messages which usually are especially important in this situation the filesystem may fill up and cause extreme long delays - if your system crashes, the most important messages may never be logged at all.

Compressed ROM Filesystem In some cases it is sufficent to have read-only access to some files, and if the files are big enough it becomes desirable to use some method of compression. To create a CramFs filesystem a special tool mkcramfs is used to create a file which contains the CramFs image.

Especially the endianness requirement makes it impossible to build the CramFs image on x86 PC host when you want to use it on a PowerPC target. The endianness problem has been fixed in the version of mkcramfs that comes with the ELDK. In some cases you can use a target system running with root filesystem mounted over NFS to create the CramFs image on the native system and store it to flash for further use. Now we write the CramFs image to a partition in flash and test it: cp test.

Everything in tmpfs is temporary in the sense that no files will be created on any device. If you unmount a tmpfs instance, everything stored therein is lost.

Ramdisks cannot swap and you do not have the possibility to resize them. Mount Parameters tmpfs has a couple of mount options: size: The limit of allocated bytes for this tmpfs instance. The default is half of your physical RAM without swap.

If you oversize your tmpfs instances the machine will deadlock since the OOM handler will not be able to free that memory. The default is half of the number of your physical RAM pages. These parameters accept a suffix k, m or g for kilo, mega and giga and can be changed on remount. To specify the initial root directory you can use the following mount options: mode: The permissions as an octal number uid: The user id gid: The group id These options do not have any effect on remount.

You can change these parameters with chmod 1 , chown 1 and chgrp 1 on a mounted filesystem. It can be found in the Filesystems configuration group.

Usage of tmpfs in Embedded Systems In embedded systems tmpfs is very well suited to provide read and write space e. Compressed ROM Filesystem. One way to achieve this is to use symbolic links. The following description does not cover such configurations. Only the solution which uses a standard PC Card Slot is described here. If no card is present you will see a message like this: PPCBoot 1. With a CompactFlash Card you get Even more exotic memory devices like the "MemoryStick as used in some Digital Cameras" will usually work without problems And with a harddisk adapter you would see Note that most other cards will be detected by U-Boot, but not supported otherwise, for instance This is a quite complex set of kernel modules and tools that take care of things like automatic detection and handling of "card insert" or "remove" events, identification of the inserted cards, loading the necessary device drivers, etc.

This is a very powerful package, but for embedded applications it has several serious disadvantages: Memory footprint - the package consists of a lot of tools and modules that take a lot of space both in the root filesystem and in system RAM when running Chicken and Egg Problem - the package loads the needed device drivers as kernel modules, so it needs a root filesystem on another device; that means that you cannot easily put the root filesystem on a PC Card.

This has the big advantage of minimal memory footprint, but of course it comes with a couple of disadvantages, too: It works only with "disk" type PC Cards - no support for modems, network cards, etc; for these you still need the PCMCIA Card Services package.

There is no support for "hot plug", i. Well, of course you can do this, but either you willnot be able to access any card inserted, or when you remove a card you will most likely crash the system. On the other hand these are no real restrictions for use in an Embedded System. If you start with a new drive, you have to start by creating a new partition table.

Notes: Base and length fields are blocks, which vary in size between media. The name of a partition is descriptive text. Is that okay? You will prefer this format if you want to exchange your "disk" media with any PC type host system.

Changes will remain in memory only, until you decide to write them. The number of cylinders for this disk is set to There is nothing wrong with that, but this is larger than , and could in certain setups cause problems with: 1 software that runs at boot time e. Calling ioctl to re-read partition table. Syncing disks.

Using PC Card "disks" with U-Boot and Linux U-Boot provides only basic functionality to access PC Card based "disks": you can print the partition table and read and write blocks addressed by absolute block number , but there is no support to create new partitions or to read files from any type of filesystem. Since U-Boot cannot read files from a filesystem you should create one or more small partitions maybe 1 MB or so if you want to boot from the "disk".

It then reads the image from disk and stores it in memory. Image Name: Linux OK Uncompressing Kernel Image OK Linux version 2. Booting image at Freeing unused kernel memory: 48k init init started: BusyBox v0. This allows the Linux kernel to skip certain parts of the framebuffer initialization and to reuse the framebuffer contents that was set up by the U-Boot firmware. This allows to have an image displayed nearly immediately after power-on, so the delay needed to boot the Linux kernel is masked to the user.

The current implementation has some limitations: We did not succeed in reusing the previously allocated framebuffer contents directly. Instead, Linux will allocate a new framebuffer, copy the contents, and then switch the display. This adds a minimal delay to the boot time, but is otherwise invisible to the user. Linux manages its own colormap, and we considered it too much effort to keep the same settings as used by U-Boot. Instead we use the "trick" that U-Boot will fill the color map table backwards top down.

This works pretty well for images which use no more than If the images uses more colors, a bad color mapping may result. We strongly recommend to convert all images that will be loaded as Linux splash screens to use no more than colors. The "ppmquant" tool can be used for this purpose see Bitmap Support in U-Boot for details. Usually there will be a Linux device driver that is used to adjust the brightness and contrast of the display. When this driver starts, a visible change of brightness will happen if the default settings as used by U-Boot differ.

We recommend to store settings of brightness and contrast in U-Boot environment variables that can be shared between U-Boot and Linux.

This way it is possible assuming adequate driver support to adjust the display settings correctly already in U-Boot and thus to avoid any flicker of the display when Linux takes over control. It is not an easy task to design the root file system for an embedded system.

There are three major problems to be solved: what to put in it which file system type to use where to store and how to boot it For now we will assume that the contents of the root file system is aready known; for example, it is given to us as a directory tree or a tarball which contains all the required files. We will also assume that our system is a typical resource-limited embedded system so we will especially look for solutions where the root file system can be stored on on-board flash memory or other flash memory based devices like CompactFlash or SD cards, MMC or USB memory sticks.

So our focus here is on the second item: the options we have for chosing a file system type and the consequences this has.


The DENX U-Boot and Linux Guide (DULG) for canyonlands

When called without a count argument, the value will be written only to the specified address. U This is intended as a special form of a memory test, since this command tries to read the memory as fast as possible. This command will never terminate. There is no way to stop it but to reset the board! The number of flash banks is printed with information about the size and organization into flash "sectors" or erase units.


Special Commands 5. St Miscellaneous command 5. Sleep-delay execution for some time 9. U-Boot environment variables 5.

Related Articles