ARM memory mapping: INT15 equivalent? Standard way to query memory map?

Question

On PC-architectures (where the presence of the BIOS and the usage of it is pretty much standardized), you can discover the size of the RAM memory, as well as its reserved/free for use regions by using the INT15 BIOS interrupt, function 0xE820.

Since I'm passionate about low-level programming and after programming Intel architectures for approximately 6 months, I decided I should try and learn how other architectures work. So I've started digging into ARM development. I've got 2 boards I'm currently working on: Olimex A20 OlinuXino-MICRO and Samsung Arndale's Exynos 5250. What I'm trying to do is to port a hypervisor I've developed for Intel architectures to these two boards. I am now in the stage of trying to programmatically detect the memory map of the system in a reliable and acceptably standardized way (I would prefer not to write entirely different code for different ARM boards). But so far, I find the relevant documentation to be a little bit confusing.

On the Olimex A20 I've got a Cortex-A7 ARM CPU. The PDF found here: http://infocenter.arm.com/help/topic/com.arm.doc.den0001c/DEN0001C_principles_of_arm_memory_maps.pdf , which applies to Cortex-A7 and other CPUs, states at page 14 that the memory addressing space from 1GB-to-2GB is reserved for Memory-Mapped I/O devices, whereas the Olimex-A20 documentation found at this link https://github.com/OLIMEX/OLINUXINO/blob/master/HARDWARE/A20-PDFs/A20%20User%20Manual%202013-03-22.pdf?raw=true states at page 21 that the memory addressing space from 1GB-to-3GB is DDR-II/DDR-III memory.

Am I simply confused or is there an inconsistency between these two documents?

Answer 1

The memory maps on ARM chips are highly chip-specific. There is also usually nothing like a BIOS, so your bootloader or hypervisor will have to figure out the memory layout on its own.

Generally you'd need to work with the SDRAM controller to query and initialize the installed SDRAM chips. This is a non-trivial and, once again, very chip-specific process. You should check the code of bootloaders (e.g. U-Boot) you have available for your chips and look for the memory init code.

However, in many of cases, the memory "map" (start of RAM and its size) is simply hardcoded for each board the bootloader is ported to, since it's unlikely to change at every boot.

Answer 2

Historically ARM boot-loaders pass information to the Linux kernel using ATAG structures as desribed in Booting ARM Linux. At a minimum the boot-loader is expected initialize the RAM in the system and pass ATAG_MEM structures to describe where the RAM lives in the address space. Interpreting these structures would give you some of the information you need but it doesn't tell you anything about any peripheral devices. In this booting method the machine type is used to trigger platform specific code to initialize the rest of the hardware.

The new way of doing this is through the Flattened Device Tree. The device tree originated with OpenFirmware and besides describing the RAM mapping can also describe the rest of the hardware and peripherals.