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
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