[U-Boot] [PATCH] fdt_support: Use CONFIG_NR_DRAM_BANKS if defined

[Vadim Bendebury]

On Tue, Apr 30, 2013 at 2:14 PM, Tom Rini <trini at ti.com> wrote:
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> On 04/30/2013 04:49 PM, Doug Anderson wrote:
>> Tom,
>>
>> On Tue, Apr 30, 2013 at 1:35 PM, Tom Rini <trini at ti.com> wrote:
>>> And I guess having this knowledge correct for the kernel is
>>> useful in other contexts like when we want to power down some
>>> banks of memory but not others?  I mean, there's "lots" of
>>> platforms that lie and say 1 bank since we require contiguous
>>> mapping.  Thanks!
>>
>> Thanks for the review!
>>
>> At the moment I'm _not_ convinced that there's a good reason to
>> specify 8 banks.  We appear to have lied and said 1 bank on
>> exynos5250-snow (ARM Chromebook) and I don't know of any bad side
>> effects.
>>
>> The code I'm looking at right now indicates 8 banks.  We need to
>> track down why someone did that but it doesn't seem totally crazy
>> to allow specifying the proper number of banks so I figured I'd
>> send this patch up.
>>
>> If you prefer, we can leave this patch hanging until we actually
>> track down if specifying 8 banks was really needed.
>
> Yes please, lets hold.  Thanks!
>

I looked into this a bit more, what happens on this particular target
(Exynos5420 with 4GB DRAM onboard) is that out of 4GB of memory only
3.5GB is usable, as the lower .5 GB of address range is taken by the
architecture, and the address bus width is 32 bits.

U-boot code makes several assumptions:
 - bank size is a power of 2
 - bank base is aligned with bank size
 - all bank sizes are the same

with this in mind, the only way to describe our memory situation is to
define 7 banks, .5GB each, the lowest one starting at 0x20000000
(.5GB).

This is not a big deal for u-boot (maybe very marginally inefficient
when determining the actual memory size). Is this a big deal for
kernel? I mean it is easy to squash these seven memory banks into one
when filling out the memory node of the device tree, the question is
is it even necessary?

cheers --vb

> - --
> Tom
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