[U-Boot] [PATCH] mpc86xx: adjust the DDR BATs after calculating true DDR size

[Becky Bruce]

On Mar 29, 2010, at 6:07 PM, Timur Tabi wrote:

> On Mon, Mar 29, 2010 at 5:09 PM, Becky Bruce <beckyb at kernel.crashing.org 
> > wrote:

<snip>

>
>
>> I think you need to adjust how much usable ram u-boot thinks it has  
>> if
>> you can't map it all.
>
> I tried that.  The problem is that U-Boot uses this number to tell
> Linux how much DDR there is.  The code doesn't really support U-Boot
> and Linux seeing different amounts of DDR.

>> If you have one BAT, and you have an amount of
>> memory that is ! a power of 2, then you're going to leave a chunk
>> unmapped, which can cause problems later.
>
> AFAIK, we always have only one BAT for DDR.  I wasn't planning on
> expanding the scope of this patch to add support for multiple BATs.

I'm not suggesting that - you really can't use multiple BATs as there  
are only 8 of them and they're a hot resource.

>
> I don't know how to handle !2^X sizes of DDR.

You can't map !2^x sizes of DDR with one BAT, so in this case you're  
always going to end up with a mapping that's smaller than the actual  
size of DDR, which is a *new* scenario for 86xx.  IIRC, before, we  
always set the BAT up for 2G, so we either 1) had *less* RAM than we  
had BAT mappings for or 2) pretended that we had 2G of memory and  
ignored the rest.  Have you looked at 83xx or 85xx to see if they do  
anything interesting here?  It's possible this problem has just been  
ignored other places, and we haven't had an issue yet.  It should't be  
too complicated to get get_effective_memsize to report something  
reasonable.

>
>>> +#define KB_TO_BATU(x) ((((x)/128) - 1) * 4) /* Convert KBs to BATU
>>> value */
>>
>> It seems somewhat arbitrary that you decided to use take KB here as  
>> an
>> arg when the BATU_SIZE macro returns bytes.   I'd prefer to see
>> symmetry here.
>
> I used KB to keep the sizes of numbers small.  The smallest value is
> 128KB, so it's not *that* arbitrary.

The other stuff used bytes.  It's confusing.  Just use bytes.....

-B