Thread: Question on embedded systems

Hi, I have been trying to learn embedded systems and i am half way through. I choosed a board called viper-lite as my development target board. This board comes along with the redboot (Boot loader) and embedded Linux and Eco's OS's.

I wrote a program which actually configures the serial port. Since I initially decided to run this code on reboot, I was apply to refer UART registers directly. But when I try the same program on embedded Linux with some changed to the code, i get a seg fault. Well thats true, because i am trying to access the kernel space without any memory map UART registers.

My question here is, will I be able to run the same code which works on red boot on Linux. Will i be able to memory map the UART registers, and use struct overlay to configure UART registers? I tried one more solution, on Linux as well, by opening the ttyS0 file and writing on to the file. Which actually works.

But just curious to know how will make the other code to work on Linux.

Thank you

ssharish2005

I expect that you are talking of a memory mapped IO register set for the serial port, which means that the address of the serial port will have changed when the paged memory mode was enabled by Linux. This in turn means two things:
1. The ACTUAL memory address of the registers is probably not what you think it is - it MAY be, but quite commonly it's "moved" (this is often done on purpose to detect direct accesses to hardware that isn't supposed to go there without the help of the memory mapping system - that is, ALL addresses are changed to something other than what it originally was, so that a direct access to the original hardware access is NOT a valid one).

2. Since all hardware "ports" are owned by the kernel, you shouldn't be able to access them even if the memory address was known to you - because only kernel mode drivers should be able to access hardware. It may be possible to create a mini-driver that just allows you to map a set of registers to user-mode - but you definitely need some kernel-mode code to create this.

There are ways around it - see this page: http://www.linuxjournal.com/article/5442

The other option is to use a proper kernel mode driver module - they aren't trivial, but once you get the hang of it, they aren't terribly hard either (until the hardware gets complicated, at least).

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Mats

hey thanks for the reply. Can i put it like this. In have got the device driver written. How will i made my user defined device driver to the kernal?

ssharish2005

You need to "register the device", which is a system call in kernel mode - you can't register devices in user-mode. I think (from 30s Google) that you want the "register_chrdev()" function - look it up.

Getting a book on Linux device drivers, such as this might be a good idea.

There's probably books on Embedded Linux that may be useful too.

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Mats