Linux – How to Reserve a Block of Memory from the Kernel

hardwarekernellinuxmemory

I have a device that needs a block of memory that is reserved solely for it, without the OS intervening. Is there any way to tell BIOS or the OS that a block of memory is reserved, and it must not use it?

I am using this device on an openSUSE machine.

Best Answer

What you're asking for is called DMA. You need to write a driver to reserve this memory.

Yes, I realize you said you didn't want the OS to intervene, and a driver becomes part of the OS, but in absence of a driver's reservation, the kernel believes all memory belongs to it. (Unless you tell the kernel to ignore the memory block, per Aaron's answer, that is.)

Chapter 15 (PDF) of "Linux Device Drivers, 3/e" by Rubini, Corbet and Kroah-Hartmann covers DMA and related topics.

If you want an HTML version of this, I found the second-edition version of the chapter elsewhere online. Beware that the 2nd edition is over a decade old now, having come out when kernel 2.4 was new. There's been a lot of work on the memory management subsystem of the kernel since those days, so it may not apply very well any more.

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Is there any method to mount a block of memory as a filesystem in linux

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(bet you'd never have guessed that). You might also want to have a look at tmpfs and overlay filesystems (latter provides a mechanism for cloning the filesystem at power up/down - see Puppy Linux)

Linux – Restricting memory area for linux kernel

To add detail to David Schwartz's answer, by specifying the kernel boot parameter to set the amount and start address used by the kernel, you can do what you want.

For example, add the following command to your kernel boot args:

mem=256M@0x80000000

Will indicate the kernel should use 256M starting at 0x80000000, which is what you require.

You should revert the CONFIG settings to their original values, and verify your kernel will boot correctly in its default configuration (with no mem= setting).

You can first try out mem= by specifying

mem=256M

which will give you a reduced memory area starting at 0, with the top 256M being unused. Once that is seen to work, use the 256M with offset as above.

Take care also where in RAM you load the initial compressed image by uBoot, to ensure it does not conflict with the eventual kernel memory area.

Best Answer

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