Re: question on remap_page_range()

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Tue, 31 Aug 1999 10:23:37 +0200 (MET DST), Gilles Pokam
<pokam@cs.tu-berlin.de> said:

>> No.  Either Rubini is wrong or you have misinterpreted.  A physical
>> address is just that --- the physical address of the memory as it
>> appears on the cpu bus when the cpu goes to read from ram.  It is
>> completely untranslated.  The first physical address in the system is
>> usually zero, not PAGE_OFFSET.  

> Sorry, i forget to said "from the kernel point of vue" :

> Rubini's book, page 274 about PAGE_OFFSET:

> " (...) PAGE_OFFSET must be considered whenever "physical" addresses are
> used. What the kernel considers to be a physical address is actually a
> virtual address, offset by PAGE_OFFSET from the real physical
> address.(..)"

That is wrong.  A physical address is a physical address is a physical
address.  Even from the kernel's point of view.  We need to know
physical addresses when we are setting up page tables, for example, and
these do NOT have a PAGE_OFFSET applied.  Ever.

However, the kernel cannot directly access the contents of a physical
address in memory: all memory accesses, without exception, go through
virtual address translation, and _that_ is where PAGE_OFFSET comes in.

Physical addresses do not, ever, include the PAGE_OFFSET bias.

It would be true to say that "What the kernel uses to *access* a
physicall address is actually a virtual address, offset by PAGE_OFFSET
from the real physical addres."  However, the kernel still calls that
latter, offset address a virtual address, not a physical address.  It's
a kernel virtual address as opposed to a user virtual address, but it is
still virtual, not physical.

> About remap_page_range Rubini said: (page 280-281)
> " remap_page_range(unsigned long virt_addr,unsigned long phys_add,
> 	          unsigned log size,pgprot_t prot);
>  unsigned long phys_add:
>     The phyical address to which the virtual address should be mapped. The
>     address is physical in the sense outline above" (in PAGE_OFFSET)

That is nonsense, the physaddr in remap_page_range does not include
PAGE_OFFSET bias.  It makes no sense at all for it to do so.

> To map to user space a region of memory beginning at physical address
> simple_region_start with size = simple_region_size he used the following
> example:
> unsigned long physical = simple_region_start + off + PAGE_OFFSET

That will work on 2.0, but only because PAGE_OFFSET is zero.  It won't
work on 2.2.

--Stephen
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