UNIX for Dummies Questions & Answers

Hi all,

I am new to Linux kernel/user space programming having been an assembly programmer in my previous life. I am now using 2.6.x kernel on an embedded CPU that has a few dedicated hardware blocks (including more CPU running just C-code, i.e., no operating system).

There is a single DRAM connected to this chip with one Linux CPU + multiple h/w blocks. No swapping since this is being done for an embedded device (SoC ASIC chip with a single 32-bit DDR2 interface, no hard-drive).

Question(s):

1. The Linux CPU needs to talk to hardware blocks that obviously physical DRAM addresses while Linux processes/threads use virtual addresses.

2. How do I translate these addresses back-n-forth? For example, a Linux process may want to allocate memory and then hand it off to a hardware block to write into it. Then after a while the process will read it.

3. Sometimes, the hardware block may write a physical address into the shared memory. The Linux CPU will read the shared memory and then convert the physical address to virtual memory and go read that location.

How does one achieve all of this? If this is being extremely stupid, then please let me know. Hopefully, you can give me some pointers. A website, book, code, man page, high-level thoughts, anything would be appreciated.

Thanks a lot,
Guraaf

Translating real addresses to physical addresses is almost always a function of the hardware. The CPU refers to a table in RAM assigning physical to virtual ranges, it's the responsibility of the OS to manage these tables. Where this table is and how it's laid out depends on the architecture.

There's easier ways to access physical RAM from userspace than making a kernel driver, though. I think you can just map in the relevant bits of /dev/core, which will give you access to the ranges you want while letting the kernel's existing facilities manage all the translation.