[ECOS] Why virtual address and Physical address are different in the memory layout of IXPD425 board ?

[ECOS] Why virtual address and Physical address are different in the memory layout of IXPD425 board ?

[Sujoy Ray]

Hello:

I was going through the memory laout of an IXPD425
board provided in the following link:-

http://sources.redhat.com/ecos/docs-latest/redboot/ixdp425.html


It has following lines.


Virtual Address   Physical Address  XCB  Description
0x00000000       0x00000000         010  SDRAM
(cached)
0x10000000       0x00000000         010  SDRAM (alias)
0x20000000       0x00000000        000 SDRAM
(uncached)

Now, I have following questions.

1. Why in the third line (SDRAM uncached), the virtual
address and physical address are different? I know
that virtual address can be different from the
physical address. But normally it is not in the
embedded scenario. 

2. Who is going to use this address? I have seen that
all the addresses are assigned in the .ld file and
those are static allocation. If this is true, then how
this virtual address will be used. 

May be 'am missing some fundamental concept. I would
be nice if anyone can help me in understanding this
concept in a better way.

Thanks in advance.

Regards,
Sujoy Ray.




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Re: [ECOS] Why virtual address and Physical address are different in the memory layout of IXPD425 board ?

[Andrew Lunn]

On Sun, Aug 08, 2004 at 11:14:03PM -0700, Sujoy Ray wrote:
> 
> Hello:
> 
> I was going through the memory laout of an IXPD425
> board provided in the following link:-
> 
> http://sources.redhat.com/ecos/docs-latest/redboot/ixdp425.html
> 
> 
> It has following lines.
> 
> 
> Virtual Address   Physical Address  XCB  Description
> 0x00000000       0x00000000         010  SDRAM
> (cached)
> 0x10000000       0x00000000         010  SDRAM (alias)
> 0x20000000       0x00000000        000 SDRAM
> (uncached)
> 
> Now, I have following questions.
> 
> 1. Why in the third line (SDRAM uncached), the virtual
> address and physical address are different? I know
> that virtual address can be different from the
> physical address. But normally it is not in the
> embedded scenario. 

Probably the fundemental concept you are missing is the difference
between cachaed an uncached memory. Device drivers, particularly PCI
windows, need to access the ram in uncached mode. Normal programs make
use of the cache because of the extra performance it gives. Any easy
way to acheive this is to have the same memory in the address space
twice, once caches and one uncached. The code can then decide which
one to access.

 
> 2. Who is going to use this address? I have seen that
> all the addresses are assigned in the .ld file and
> those are static allocation. If this is true, then how
> this virtual address will be used.

The specific device drivers will know about this second mapping. It
will probably be hard coded somewhere.

        Andrew

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Re: [ECOS] Why virtual address and Physical address are different in the memory layout of IXPD425 board ?

[Sujoy Ray]

Andrew,

Thanks for your suggestion. It really helped me to get
the concept. Apart from what you told, doese this
mapping has any relationship with LINUX booting ?

Regards,

Sujoy




--- Andrew Lunn <andrew@lunn.ch> wrote:

> On Sun, Aug 08, 2004 at 11:14:03PM -0700, Sujoy Ray
> wrote:
> > 
> > Hello:
> > 
> > I was going through the memory laout of an IXPD425
> > board provided in the following link:-
> > 
> >
>
http://sources.redhat.com/ecos/docs-latest/redboot/ixdp425.html
> > 
> > 
> > It has following lines.
> > 
> > 
> > Virtual Address   Physical Address  XCB 
> Description
> > 0x00000000       0x00000000         010  SDRAM
> > (cached)
> > 0x10000000       0x00000000         010  SDRAM
> (alias)
> > 0x20000000       0x00000000        000 SDRAM
> > (uncached)
> > 
> > Now, I have following questions.
> > 
> > 1. Why in the third line (SDRAM uncached), the
> virtual
> > address and physical address are different? I know
> > that virtual address can be different from the
> > physical address. But normally it is not in the
> > embedded scenario. 
> 
> Probably the fundemental concept you are missing is
> the difference
> between cachaed an uncached memory. Device drivers,
> particularly PCI
> windows, need to access the ram in uncached mode.
> Normal programs make
> use of the cache because of the extra performance it
> gives. Any easy
> way to acheive this is to have the same memory in
> the address space
> twice, once caches and one uncached. The code can
> then decide which
> one to access.
> 
>  
> > 2. Who is going to use this address? I have seen
> that
> > all the addresses are assigned in the .ld file and
> > those are static allocation. If this is true, then
> how
> > this virtual address will be used.
> 
> The specific device drivers will know about this
> second mapping. It
> will probably be hard coded somewhere.
> 
>         Andrew
> 


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Re: [ECOS] Why virtual address and Physical address are different in the memory layout of IXPD425 board ?

[Andrew Lunn]

> Thanks for your suggestion. It really helped me to get
> the concept. Apart from what you told, doese this
> mapping has any relationship with LINUX booting ?

I've no idea, but i don't see any reason why it should.

        Andrew


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