MPC8260 strange virtual addresses returned by kmalloc()

Laurent Lagrange lagrange at
Fri Jan 7 04:42:50 EST 2005

Hi Eugene

My PAGE_OFFSET is defined as follow :
#define CONFIG_KERNEL_START (0xc0000000)     in

After reading kmalloc manual pages, I am in doubt.
My call is like
if ((tmpbuffer = kmalloc(128 * 1024, GFP_KERNEL)) == NULL){

This is the first time, I kmallocate a 128KB buffer which seems to be the
contiguous memory space which can be allocated.
Is it true on PPC ? Some PPC drivers seems to only allocate multiple
PAGE_SIZE (4KB) buffers.

My allocated buffer is intended to be used for DMA accesses.
Must I add the GFP_DMA flag to force the kmalloc to use contiguous memory ?

The Linux code for vmalloc is based on successive kmalloc calls, but I don't
what the kmalloc do when the desired size is greater than the PAGE_SIZE.

What is the right way to allocate big (128KB ???) DMA buffers ?
The manual pages say "Only for DMA kmalloc or __get_dma_pages() has to be

The global variable is well in the loaded module.
OK, I use a kmalloc for the buffer instead of using the global variable.

Thanks for your ideas.

-----Message d'origine-----
De : Eugene Surovegin [mailto:ebs at]
Envoye : mer. 5 janvier 2005 18:42
A : Laurent Lagrange
Cc : linuxppc-embedded at
Objet : Re: MPC8260 strange virtual addresses returned by kmalloc()

On Wed, Jan 05, 2005 at 02:38:12PM +0100, Laurent Lagrange wrote:
> Hi and Happy New Year to all,
> I use a Linux 2.4.25 kernel on a MPC8260 custom board with 128MB of RAM.
> PCI is activated and my drivers are loaded as modules.
> The RAM is mapped at 0xC0000000 as usual.
> On MPC8XX I used kmalloc and iopa to get the physical addresses of the
> allocated buffers.
> On MPC82XX I used kmalloc and __pa for the same thing and it worked fine
> with previous kernel.
> But now, when I want to use kmalloc, I get addresses like 0xC9xxxxxx.
> The __pa gives me physical addresses 0x09xxxxxx which are out of my
> RAM.
> I tried old iopa function and got some real physical addresses and the
> module works.
> The virt_to_bus gives me too addresses like  0x09xxxxxx
> (ie substract kernel base address 0xC0000000 as usual).
> I don't understand why I get virtual addresses which seem to be out of my
> memory and why it runs.

Yeah, this looks strange, please, check that this address is really
from kmalloc and not from vmalloc.

Also, check that PAGE_OFFSET is really 0xc000'0000.

> More, I have a global variable which is a little buffer. It is also mapped
> at a 0xC9xxxxxx address.

Is this global variable in the loaded module? If yes, this is normal,
because modules are loaded into vmalloced space and you cannot use
__pa for such addresses.


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