starting RAM adress for linux kernel

starting RAM adress for linux kernel

[scarayol]

Hello,

We have a platform similar to MPC885ADS (xith Kernel version 2.4.26) except
that :
- the RAM is at 0x0300 0000 (size 0x10000000)
- the flash is at 0x0280 0000 (size  0x00200000)
- IMMR = 0x0220 0000

After u-boot on MPC885ADS, we get the following platform information :
- RAM at 0 size 0x0080 0000
- FLASH at 0xFE00 0000  size 0x00200000
- IMMR at 0xFF00 0000

This information is different from the documentation of MPC885ADS :
- FLASH at 0x0280 0000  size 0x00800000
- IMMR at 0x0220 0000


1) in fads.h of u-boot, it is specified that RAM __must_ start at 0 : it
isn't true on our board :
- can linux work with RAM not beginning at 0 on this board ?
- what modifications should we make in the kernel to accept RAM at 0x0300
0000 ?

2) is brd_info the only structure given to kernel by u-boot ?
That is to say is it the only way for the kernel to have information about
the boad  ?

4) are the following values important if  CONFIG_HIGHMEM is not defined
(they are automatically generated by our Metrowerks PCS tool in autoconf.h
) ?
#define CONFIG_HIGHMEM_START 0xfe000000
#define CONFIG_LOWMEM_SIZE 0x30000000
#define CONFIG_KERNEL_START 0xc0000000
#define CONFIG_TASK_SIZE 0x80000000
Should we change these values  if  CONFIG_HIGHMEM is not defined ?

5) is the base value of IMMR very important (real value in u-boot different
from documentation) ?

Thanks really for your help.

Sophie CARAYOL.

Re: starting RAM adress for linux kernel

[Wolfgang Denk]

Dear Sophie,

in message <OFED9D0639.75930571-ONC1257015.0031274C@brime.fr> you wrote:
> 
> We have a platform similar to MPC885ADS (xith Kernel version 2.4.26) except
> that :
> - the RAM is at 0x0300 0000 (size 0x10000000)
> - the flash is at 0x0280 0000 (size  0x00200000)
> - IMMR = 0x0220 0000

Ummm - just to avoid any misunderstandings:  there  is  no  technical
reason for such a memory map, and no other good reason either as such
a configuration will not be able to run Linux.

> After u-boot on MPC885ADS, we get the following platform information :
> - RAM at 0 size 0x0080 0000
> - FLASH at 0xFE00 0000  size 0x00200000
> - IMMR at 0xFF00 0000
> 
> This information is different from the documentation of MPC885ADS :
> - FLASH at 0x0280 0000  size 0x00800000
> - IMMR at 0x0220 0000

So what is your problem? The memory map ist just something you define
as it fits your project. The values in the MPC885ADS are  not  really
useful  for  any  practical  purposes,  so  U-Boot  choses  to define
different, more useful values. This is perfectly fine.

> 1) in fads.h of u-boot, it is specified that RAM __must_ start at 0 : it
> isn't true on our board :

This applies for your board, too.

> - can linux work with RAM not beginning at 0 on this board ?

You can make it work, but there is no good reason to  do  this.  Juts
change your memory map to use a more sensible configuration.

> - what modifications should we make in the kernel to accept RAM at 0x03000000 ?

Please forget this idea.

> 2) is brd_info the only structure given to kernel by u-boot ?
> That is to say is it the only way for the kernel to have information about
> the boad  ?

It also gets passed the kernel commandline and  some  other  data  in
registers.

> 4) are the following values important if  CONFIG_HIGHMEM is not defined

You don't want to play with these data unless you know  exactly  what
you are doing.

> (they are automatically generated by our Metrowerks PCS tool in autoconf.h ) ?

Then please ask Metrowerks support who provided such a tool which  is
unknown to me.

> 5) is the base value of IMMR very important (real value in u-boot different
> from documentation) ?

Yes, it is very important. Your current memory map cannot  work  with
Linux.

Best regards,

Wolfgang Denk

-- 
Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd@denx.de
Fascinating is a word I use for the unexpected.
	-- Spock, "The Squire of Gothos", stardate 2124.5