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

[Tom Rini]

On Wed, May 15, 2013 at 08:58:03AM -0700, Vadim Bendebury wrote:

> 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?

OK, this would be the second case of needing to describe the memory in
the DT in a way that conflicts with how we dynamically do the node.
Lets go and try again at a patch that lets boards opt-in to "do not
override the memory property, it was already correct and you broke it".

-- 
Tom
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