[U-Boot] Enabling Cache in SPL - how to speed up booting from SPL

[Stefano Babic <sbabic@denx.de>]

On 10/02/2012 13:28, Marek Vasut wrote:
>> Hi,
>>
>> because we are fast able to boot the kernel directly from SPL, I am
>> experimenting now how we can speed up the process. Mainly my focus is
>> how to make copy from storage (=NAND) to RAM faster.
>>
>> There are two approaches I can see: using DMA in NAND driver, or/and
>> enabling cache. I have not yet patches (I hacked the code dirtly), but I
>> have some results I cannot understand.
>>
>> First of all, DMA. DMA for OMAP is mainlined now with Simon's patches,
>> enabling DMA in nand_spl_simple.c as sent by Simon does not work for me,
>> but I have adapted the patch to my platform (twister board, mainlined,
>> with TI AM3517). Here the results measuring between start of the copy
>> and before giving the control to the kernel (kernel size is 2188100):
>>
>>                                    Seconds
>> -----------------------------------------------------------------
>> No cache, no DMA                    1.700
>> DMA in NAND driver                  1.190
>> Enabling Cache                      2.120
>>
>> With enabled DMA I won on my platform ~600mSec. However, enabling cache
>> is worse than disabling, and this let me really confused.
>>
>> I have not done any special things to enable cache, and I reused code
>> already provided. I have only set to an arbitrary address in RAM (but
>> 64KB aligned) gd->tbl, and then I called enable_caches(). I can see that
>> mmu_setup() is called, and at the end icache_status() and
>> dcache_status() return both that cache is enabled.
>>
>> But copying from NAND takes longer enabling cache, and this does not
>> really make sense. Does anybody have an hint what I am missing ?
> 
> L2 is on too?

As far as I have understood, I can disable L2 on TI-omap3 only if I set
CONFIG_SYS_L2CACHE_OFF, and this is not my case. And there should be
then any apparent difference between SPL and U-Boot by enabling the
cache (I mean, if L2 is off, is off also in u-boot). When U-boot runs, I
see only that enable_caches() is called after setting the page tables,
and that is enough in u-boot to get dcache working. I did the same in
SPL, but it seems really something is missing...

Stefano

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