Subject: Virtual addresses, ioremap, vmalloc, etc

From: [email-masked] (Mason)
Date: Tue, 1 Dec 2015 13:08:09 +0100

Hello everyone,

I was wondering if someone could help clear my confusion.

In my company's legacy port (based on 3.4, dating back to 2.6) someone
chose to map the first 16 MB of physical addresses using:

static struct map_desc tango_map_desc[] __initdata = {
	{
		.virtual	= 0xf0000000,
		.pfn		=__phys_to_pfn(0),
		.length		= SZ_16M,
		.type 		= MT_DEVICE,
	},
};

static void __init tango_map_io(void)
{
	iotable_init(tango_map_desc, ARRAY_SIZE(tango_map_desc));
}

Is the virtual address 0xf0000000 chosen arbitrary?
Could I pick 0xf04200000 for example?

The same kernel, with no such boot-time mapping prints:

[    0.000000] Memory: 641720K/655360K available (3135K kernel code, 109K rwdata, 1056K rodata, 3044K init, 218K bss, 13640K reserved, 0K cma-reserve)
[    0.000000] Virtual kernel memory layout:
[    0.000000]     vector  : 0xffff0000 - 0xffff1000   (   4 kB)
[    0.000000]     fixmap  : 0xffc00000 - 0xfff00000   (3072 kB)
[    0.000000]     vmalloc : 0xe8800000 - 0xff000000   ( 360 MB)
[    0.000000]     lowmem  : 0xc0000000 - 0xe8000000   ( 640 MB)

It looks like 0xf0000000 is in the middle of the vmalloc space.
Is it a good idea to "statically" map something there?

If I were to call ioremap(0, SZ_16M); at run-time, I would imagine
the virtual address could be anywhere in the vmalloc space?
There's no reason it would be 0xf0000000, right?

In short, is virtual address 0xf0000000 special in any way?
(Other than being in the vmalloc space perhaps.)

For my own reference:
https://www.kernel.org/doc/Documentation/arm/memory.txt

Regards.