How does the uboot gets loaded for a particular embedded linux device? Is there a program inside the chip that searches for uboot inside the sd card or it is some another concept?
You just need to do a bit of Google-ing to find this out. Take a look at http://www.linuxjournal.com/content/handy-u-boot-trick for example. Yes, in short, the ROM knows how to read a FAT32 partition, typically, and loads and executes uboot.
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I am working on a project with SAMA5D3-xplained board with CortexA5 processor and embedded Linux. I would like to send and receive some data via UART during U-Boot is running and before a kernel is loaded to the RAM. I have no idea what I should do. Should I add the second UART to U-Boot device tree source file? Should I change something in a board configuration file? Do you have any ideas on what steps I should take to achieve my goal? Thank you in advance for any help.
EDIT
I would like to use UART from U-Boot C code, not from U-Boot commands. I need to communicate with one of a peripheral device before the kernel is loaded to the RAM.
I assume that you are using the upstream U-Boot from https://source.denx.de/u-boot/u-boot.git.
The device-tree arch/arm/dts/at91-sama5d3_xplained.dts already contains the definitions for six different uarts called serial0 - serial5. serial0 is set in the /chosen node as the standard serial connection. You should be able to see all six devices with the 'dm tree' command.
The currently used UART can be switched by setting the stdin and stdout environment variables.
If you do not want to switch these variables, because you still want output on the default UART you will have to access the device driver. Unfortunately drivers/serial/serial-uclass.c does not yet export functions for this. But _serial_putc(), __serial_getc(), and __serial_tstc() should give you an idea how this is done.
I am trying to develop an operating system. I have finished the Barebones tutorial from OSDev but still have some issues with multiboot.
As I understand, when I create a multiboot image following the tutorial, I have an ELF format binary file. This ELF executable is loaded into memory by QEMU when I specify the -kernel option. So I have a single executable program in memory and it has an entry point. It has some functions and libraries alongside it. There is nothing else in the memory. There is also no floppy or disk image from which I can read or write sectors. This is mainly because I haven't specified anything to QEMU in that regard.
However, what I want to achieve is after QEMU loads my ELF executable, I want to have a disk or floppy architecture from which I can read or write sectors. I also want to load some user programs from it or maybe set up a file system on it. So far, what I'm think is this:
qemu-system-i386 -kernel myKernel.bin -fda myDisk.image
Or maybe, I should take my myKernel.bin file, pad it with zeros until it is aligned to 512 bytes, then append the rest of the disk image to it.
My question is how can I achieve this functionality with QEMU. I need to know whether a task like this is possible before diving into writing a disk driver. If not possible, I can take a different approach but these are the ideas I have right now. I have already looked into -initrd and it isn't quite what I want. I also don't want to have any files loaded into the RAM by QEMU. Instead, I want to manipulate the disk image myself. Any suggestions on how to do that?
You don't need both a floppy image and a multiboot kernel, unless you've written some kind of loader. The -kernel flag works similarly to grub and it's much faster than loading from virtual media. This is how many linux kernel and driver devs test their code rather than creating a virtual media image every time.
I'm having a bit of a hard time fully understanding how the kernel starts in linux. I'm a wince developer and our company decided to run with linux instead now.
We outsourced all of the board bringup and the package I recieved is quit a bit different for the prototype board we have compared to the nitrogen6x we have been using.
Before i start listing the differences for the distro we created, the kernels are identical. The distro we have been using is a busybox system. The one we recieved from the vendor is sysvinit. I removed mdev from busybox and we are only using udev.
when I use the kernel on our busybox build the touch screen drivers breaks, or doesn' run, or does something totally magical. I'm not quit sure... there is a /dev/input/event0 device which when run on the sysvinit side is a touch device.. Is the kernel not the mechanism that binds the built-in drivers to a device node? I thought udev was for more dynamic events in the system.
On the other hand I can't really tell whats been loaded on my device. Is there a way to list running drivers that were built into the kernel? my touch pad is up? This is a fairly simple process of looking at the registry on wince to see which devices were loaded.
I guess what I'm really trying to discover, isn't so much how to add a driver to the kernel, its how the whole thing gets is plumbed together. I've found plenty of documents on createing kernel modules, but i haven't found a good resource on how to pull everything together from scratch so you can actually use said modules. Going back to the example of the touchscreen driver, its built into the kernel, how does that get plugged into /dev/input/event0??
I'm kind of having a difficult time finding good resources mostly because searching google for varations of linux/drivers/device nodes/ piles in tons of random crap from everywhere.
What you probably want to use now is evtest. It will allow you to know what are the input devices that are present and ready to use on your system.
To get more information on the input subsystem and more generic information on how the kernel is working, I can direct you to our training materials. The materials are free to download, use and redistribute.
The general answer is, there is no single, easy place to look to discover what drivers have been loaded by the kernel if they are compiled in. Of course, lsmod will display any drivers that were dynamically loaded after kernel boot.
The kernel does not create device nodes. That is, to quote your question, the kernel does not "bind" the driver to the device node. The association between kernel driver and device node is contained in the major and minor numbers registered when the driver is initialized. You can have a device node on your file system for which there is no corresponding driver (common especially in older devices where device nodes were statically created on the file system) and you can also have a driver installed for which there is no device node.
Modern Linux distros have dynamically created device nodes created on a mount point called /dev and this is usually a tmpfs file system, meaning it is volatile - it gets destroyed on every boot and recreated dynamically on each new boot.
udev is the magic that creates most device nodes based on events that it receives from the kernel when a new device is discovered (this can be after boot on device plugin, like a USB disk) or on startup when udev reads the queued events and acts on them. As you noted, busybox has a limited udev implementation called mdev.
Study udev and you will get a much better understanding of the process. Hope this helps a little.
I am new to linux kernel and Try to understand booting of Linux kernel from the point it loaded into RAM,I would like to know after Linux image loaded into RAM ,How control is passed to this image ,what all are necessary parameter needs to pass to kernel and can we pass control to linux image without passing any parameter,
I am looking into the UBOOT code with "bootm.c" but unable to understand where control is passed to Linux image,which function is responsible for it.
Is load_zimage() is responsible for passing the control/
Can anybody lead me to right direction or suggest some good tutorials on this particular part
of linux booting from x86 archetectiure.
I think it depends. Different kinds of CPU architecture, they use different ways to pass information to Linux Kernel. Of course, the Linux Kernel can boot up successfully without bootloader pass information to it, but it need to statically set up correctly in the Linux Kernel, such as root device name, console device, mem size, and also some parameters to enable/disable some features in Linux Kernel.
Why bootloader need to pass various information(parameters) to Linux Kernel, I think it's flexible consideration. Think about this case that it's possible to share one Linux Kernel on two board with same CPU but different peripheral modules.
Let me show some examples that UBoot passes information to Linux Kernel:
(1) For PowerPC cpu, nowadays they use DTB(Device Tree Blob) file to pass more information from UBoot to Linux Kernel. They consider UBoot and DTB as firmware, and in Linux Kernel, they adopt one open firmware(OF) infrastructure. You may know "bootm" command in UBoot, bootm can have three parameters, the first is uImage address, the secondary one is initrd address, and the third one is the dtb address.
(2) Earlier days, they use bootargs to pass information to Linux Kernel. Also you may know there is gd/bd structure in the UBoot, they also can pass information to Linux Kernel. But the information passed in this way is limited, not like DTB.
Hope the above information help you to understand your question.
I'm looking for a method of reading/writing (actually programming) the Master Boot Record (or maybe VBR) of a usb mass storage device. Actually its a flash drive whose MBR I want to program, so that whenever I plug it into any computer, a program (stored on my flash drive in a file) gets executed.
I know quite a bit of assembly but I don't know how to go about programming MBRs..
Please help
Thanks
If you're using linux, you can simple open the device file /dev/sdXY and write to it, the MBR will be the first 512 bytes.