[U-Boot] [PATCH 1/3 v2] 83xx: Default to using DMA to initializeSDRAM

[Jerry Van Baren <gerald.vanbaren@ge.com>]

Liu Dave-R63238 wrote:
>> According to Ira, the DMA method was faster than the cpu method:
>> "It makes the DMA initialization normal speed again. The DMA 
>> in the for loop takes the longest (as expected).
>>
>> So yes, strangely it (enabling the icache) makes a HUGE 
>> difference. The total time is <3 seconds now. It is now 
>> faster than the previous CPU method."
>>
>> Logically the DMA method should be faster, and Ira's results 
>> seem to reinforce this.  I don't have an 83xx board to test 
>> on, so let me know if others have different results than Ira.
> 
> I did the DMA init test with the orignal DMA code. At that time
> The DMA method is slower than CPU method. Strange why the
> result is different.

Instruction cache enabled/disabled is the difference - it eliminates 
instruction fetches from competing with the DMA engine on the SDRAM bus. 
  See my recent message:
<http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/63152/focus=63308>

FWIIW, in our experience with various PowerPC processors, using the 
CPU's "dcbz" (data cache block set to zero) instruction is also very 
fast, but it only works if you want to initialize memory to zero.  You 
need to enable data caching (obviously) and not having instruction cache 
enabled would cripple that method too.

[snip]

Best regards,
gvb