[U-Boot] [PATCH] ARM: tegra: Use mem size from MC in combination with get_ram_size()

[Stephen Warren <swarren@wwwdotorg.org>]

On 10/02/2014 09:38 AM, Marcel Ziswiler wrote:
> On popular request this now completes the Warren's work started for
> TK1:
>
> aeb3fcb35956461077804720b8a252d50758d7e0

Please mention the commit description too, so people can still find the 
commit if it's been cherry-picked to a different branch with a different 
commit ID:

aeb3fcb35956 ARM: tegra: Use mem size from MC rather than ODMDATA

> In addition to the move of using the Tegra memory controller (MC)
> register rather than ODMDATA for T20, T30 and T114 as well it further
> uses the generic get_ram_size() function (see "common/memsize.c")
> <supposed to be used in each and every U-Boot port>TM.

I still think calling get_ram_size() is completely pointless given how 
Tegra works and that this code is already running from RAM. Equally, 
there doesn't appear to be any protection in the implementation against 
trashing the code that implements get_ram_size() with the RAM writes the 
code performs. I'd rather not call get_ram_size() at all.

 > Added benefit is
> that it should <catch 99% of hardware related (i. e. reliably
> reproducible) memory errors> as well.

That's *extremely* unlikely. Perhaps it will catch a very tiny 
percentage of HW problems, and some number of SW configuration problems 
(i.e. BCT EmemCfg field mismatch with the board's actual RAM size). 
get_ram_size() doesn't perform anything remotely like a complete RAM 
test; it just writes to each power-of-two address, which skips a lot of 
RAM cells! Even then, I'm not sure that it's robust. One issue I 
immediately saw: it writes to *base then reads it back with only a CPU 
synchronization between and no bus clear operation (e.g. writing a 
different pattern to a different address). I've such tests pass that 
operation when there's nothing connected to the bus at all, due to 
capacitance in bus drivers/input buffers.

Aside from that, this change looks reasonable at a quick glance, 
although I do hope your testing was very thorough. I personally don't 
have time to go through and test every board we support with this change.

(As an aside, I learned something from this patch; I'd thought that our 
earlier SoCs didn't have a register in the EMC that indicated the RAM 
size, and so code *had* to use the ODMDATA to determine the RAM size. 
Evidently that wasn't the case, and we should have been using an EMC 
register all along).