[U-Boot] arch/powerpc/lib/board.c:get_effective_memsize() for 4GB+ systems

Robert Thorhuus robert.thorhuus at ericsson.com
Fri Sep 21 13:54:38 CEST 2012

> -----Original Message-----
> From: Tabi Timur-B04825 [mailto:B04825 at freescale.com] 
> Sent: den 21 september 2012 13:37
> To: Robert Thorhuus
> Cc: Kumar Gala; u-boot at lists.denx.de
> Subject: Re: [U-Boot] 
> arch/powerpc/lib/board.c:get_effective_memsize() for 4GB+ systems
> Robert Thorhuus wrote:
> > I really see the advantage of keeping this map you have. It is of 
> > course more compatible with different OSEs and you do not 
> need to do 
> > anything special with 32/64 bit cores. But at the same time 
> you never 
> > take advantage of a 64 bit core with this approach.
> U-Boot is a boot loader, not an operating system.  What is 
> U-Boot supposed to do with more than 2GB of RAM?
> > How should I access my 24GB in U-Boot?
> You should not!
> > So really you are saying that I should have a 2GB map window in my 
> > 32-bit address space and then move this window depending on what 
> > memory I want to access? A bit cumbersome I must say. But ok.
> Again, you're doing the wrong thing with U-Boot.  It's a boot loader. 
> It's supposed to find your OS, load it into memory, and then boot it.
> > How about the U-Boot relocation?
> > As I see the code, it is not easy to decide where it should 
> relocate.
> > It will be either relocated to end of RAM or if we have 
> more than 4G 
> > it will be at 4GB end.
> It relocates to the end of RAM or the end of 2GB, whichever 
> is lower.  It ignores all memory above 2GB.
> > What if you want to place U-Boot at 16MB for instance? Or if you do 
> > not want a memory map hole at 4GB just because U-Boot 
> doesn't handle 
> > more than that?
> Again, you're missing the point about U-boot.
> > Last AND least I just want to say I'm used to creating 
> memory maps in 
> > bootloader which holds for the OS. It seems the time has come to 
> > annihilate my illusion...
> Yes, please kill it with fire!
> --
> Timur Tabi
> Linux kernel developer at Freescale

Hello Timur!

You really used the machine gun there ;)

Ok. I'll just answer your 2GB+ usage question:

No I will not be using much memory at all for functionality. But the memory needs to be tested. What is your proposal for that then? 
And I see U-Boot as the first software place for test and debug. Maybe I want to read out RAM contents?


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