Physical address

[moi toi]

Hi

I am a newbie in the development of Linux driver. I
have some
difficulties to understand how the memory management
works.

I am working on a Pentium IV ( 512M of RAM), with the
Red Hat 9.0.
I want to create buffers in the RAM which are
available for DMA
transfer, and I want that process can map them.

I reserve at boot time some space in the RAM
(mem=400M).
And then I remap a buffer into the driver with the
following command: 

>unsigned long Ram_Buffer_addr;
>#define       POSITION 0x19000000 
//400*1024*1024=400M
>#define SIZE  8*1024
>
>Ram_Buffer_addr = (unsigned long) ioremap (POSITION,
SIZE);

The addresses of the buffer are the following: 
Ram_Buffer_addr               = 0xD9DCB000
Virt_to_phys(Ram_Buffer_addr) = 0x19DCB000
Virt_to_bus(Ram_Buffer_addr)  = 0x19DCB000

The virtual address is of course different from the
physical address,
and the physical address and the bus address are the
same, because I m
working on a PC. But I don t understand why the
physical address is
different from the one I gave to the function ioremap.

I did a second test: I change the position of the
buffer instead of
taking it at the address 0x19000000, the buffer start
at the address:
0x1f400000 (500M).

>unsigned long Ram_Buffer_addr;
>#define       POSITION 0x1F400000 
//500*1024*1024=500M
>#define SIZE  8*1024
>
>Ram_Buffer_addr = (unsigned long) ioremap (POSITION,
SIZE);

The addresses of the buffer are the following: 
Ram_Buffer_addr               = 0xD9DCB000
Virt_to_phys(Ram_Buffer_addr) = 0x19DCB000
Virt_to_bus(Ram_Buffer_addr)  = 0x19DCB000

The addresses are exactly the same. I m ok for the
virtual addresses,
but it sounds pretty weird for the physical and bus
addresses, they
shouldn t be the same than in the first test.

----------------------------------------------------------------------
When I map the buffer from a process, I use the
virtual address of the
buffer with the function mmap, but in the mmap
call-back function in
the driver, I use the true physical address with the
function:
remap_page_range( vma,  vma->vm_start,
POSITION,(vma->vm_end -
vma->vm_start), (pgprot_t) vma->vm_page_prot);
And it seems work! 
But if instead of POSITION, I set
Virt_to_phys(Ram_Buffer_addr), it
doesn t work anymore.

Does that mean that the functions virt_to_phys and
virt_to_bus don t
work on virtual addresses? Does anyone know, how to
get the real
physical address of the buffer.

Thanks

Francois.

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