[U-Boot] Skipping relocation RAM to RAM, esp. on i.MX6?

[Aneesh V <aneesh@ti.com>]

On Thursday 09 February 2012 05:14 PM, Wolfgang Denk wrote:
> Dear Aneesh V,
>
> In message<4F33614D.8020904@ti.com>  you wrote:
>>
>>> What exactly are you talking about here that "was adding a
>>> considerable delay" - the memory copy ?  Are you really sure about
>>> that?
>>
>> I didn't measure it part by part, but removing relocation gave a
>> noticeable speed-up, this platform is several orders of magnitude
>> slower than the real silicon. So, that should not be surprising.
>
> Could you please start using exact terminology, so we understand what
> you actually refer to?  Did you really remove the _relocation_, i. e.
> link for a static address, or did you just skip the memory copy?  Note
> that the latter should be a no-op anyway if you just load the image to
> the resulting target address.

I defeated relocation by passing to the relocate_code() function the
same address as it is linked to. I patched up arch/arm/lib/board.c for
this and fixed up the relocate_code() to correctly handle this special
case. So, relocate_code() does only .bss init now.

>
>> Maybe, I should stop the arguments now and wait till that framework is
>> a reality.
>
> I am very much convinced that you are tracking down a red herring.  It
> does not really matter if you run the code on real silicon or in an
> emulation - the relative times will always be the same.  Without any
> detailed timing analysis I simply do not believe you that you really
> have found a hot spot.  You focus on it because you found out that it
> exists and you think it was "not needed" in your configuration -
> without spending time on real optimization.

Please note that our bootloaders and kernel are customized and scaled
down for this environment. For instance, u-boot doesn't load the kernel
from network or a memory device. The kernel is preloaded in the modeled
memory for it. So, u-boot was just used to jump to the kernel. As such,
the u-boot run-time is now more dominated by pure software stuff such
as relocation. The relative timing doesn't quite apply.

>
> This is a fundamentally broken approach, and it will remain to be
> broken even if new concepts get implemented that may make it easier to
> skip certain steps of the initialization.
>
> If you are concerned about boot time optimization, you _must_ start
> with timing measurements.  You know where premature optimization leads
> to, don't you?

As I mentioned earlier boot-time is not my key care-about. Even on an
emulation platform I will probably try SPL Linux boot next time. My key
concerns are about the other aspects I mentioned, namely avoidable
complexity and problems with debugger.

br,
Aneesh