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

[Kumar Gala]

On Sep 21, 2012, at 6:54 AM, Robert Thorhuus wrote:

>> -----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:
> 
> Testing!
> 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?


Robert,

We have seen some cases for u-boot to access >4G of memory directly.  However, the effort to get u-boot to be a 64-bit clean just has never warranted the investment for the few minor cases people have raised for wanting to address >4G directly.

The one case I usually hear about is DDR test.  We suggest its easier to try and getting DDR testing to utilize a window scheme into memory than it is for one to go make u-boot 64-bit.

:)

- k