Re: Memory mapping confusion

[Rene Herman <rene.herman@xxxxxxxxxxxx>]

On 28-04-08 07:59, Chetan Nanda wrote:

> I have a few basic question regarding mapping of memory (and other
> device) in Linux kernel.
> suppose if I have 32bit machine with more then 4GB of RAM , and also
> there are other devices that are memory mapped, so how will linux
> managed to access the devices (other then RAM) that are memory mapped?

Not all of memory is (permanently) mapped into the 32-bit address-space. The 
 "more then 4GB" isn't even the main issue here: "more than 4G - 
PAGE_OFFSET - VMALLOC_RESERVE" is -- more than 896M with the standard values.

Please see http://mail.nl.linux.org/kernelnewbies/2008-04/msg00078.html

ioremap() is one of those "friends" it speaks of of vmalloc() which uses the 
VMALLOC_RESERVE window for creating mappings.

> Is RAM (in PCs) always mapped to physical location 0 ?

Physically, yes. The direct map Linux has of (the first 896M of) it is at 3G 
virtual though.

> How kernel identifies which device is at which address in entire 4GB
> address space?

The bus code enumerates devices -- to get an actual mapping of the addresses 
 the bit of hardware uses and which _it_ specifies in physical space (well, 
bus-space, but same thing on x86) you call ioremap() to map that region into 
virtual memory after which you can access it.

Rene

--
To unsubscribe from this list: send an email with
"unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx
Please read the FAQ at http://kernelnewbies.org/FAQ