[U-Boot] [RFC] ARM: U-boot and 2 GiB of ram with get_ram_size only being long

[Oliver Schinagl]

On 10/19/13 01:25, Scott Wood wrote:
> On Fri, 2013-10-18 at 18:25 -0500, Scott Wood wrote:
>> On Sat, 2013-10-19 at 01:07 +0200, Oliver Schinagl wrote:
>>> On 10/18/13 18:43, Scott Wood wrote:
>>>> On Fri, 2013-10-18 at 02:04 +0200, Oliver Schinagl wrote:
>>>>> So now that that's settled, anything fundamentally wrong with my patch? :)
>>>>
>>>> Did you see my other mail in this thread?  This patch is sort of OK for
>>> Sorry I did and I got distracted from it.
>>>
>>>> raising the get_ram_size() limit from 1 GiB to 2 GiB (with an increased
>>>> risk of false positives from I/O), but it can't go beyond that on
>>> I'd ask 'how so' but I'm not sure I'd understand anyway ;)
>>
>> Do you mean why it can't go beyond 2 GiB?  The next address to probe
>> after 0x8000_0000 would be 0x1_0000_0000 which is beyond what can be
>> addressed in a 32-bit environment.  I suppose you could return 4 GiB if
>> 0x8000_0000 tests OK, but nothing beyond that.  You'd need a larger
>> datatype than "unsigned long" if you want to return 4 GiB, though.
>
> Oh, and if you actually had 4 GiB of RAM mapped in a 32-bit environment,
> where would I/O go?
Well you need 4 GiB of address-space don't you? If you have 2 GiB of 
ram, the other 2 GiB is used as register-space isn't it. So to support 2 
GiB of ram you need 4 GiB of address sapce. Granted having more then 2 
GiB of ram is highly unlikly as you probably can't get ramsizes that 
would fit. But the bug exists with 2 GiB, get_ram_size in its current 
form, which triggered me into fixing it. get_ram_size reports negative 
ram on our 2 GiB board. Obviously incidentally it doesn't hugely matter 
because ramsize = get_ram_size(); you put a long inside an unsigned long 
and it gets automagically fixed.

My point was simply get_ram_size is bugged, it cramps signed long into 
an unsigned long, and I tried to fix that. A side effect is that upto 4 
GiB of address space is fixed. :)

Oliver
>
> -Scott
>
>
>