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

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

Jerry Van Baren wrote:
> 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>

Oops, that isn't going to be exactly correct since we aren't running out 
of RAM yet so instruction fetching isn't going to compete on the SDRAM 
bus directly.  I stand by the general theory, however, that it is bus 
contention of some sort - I would speculate the arbitration of the local 
bus to flash vs. the DMA engine to SDRAM.  Logically they should be 
independent, but they may not be due to internal interference (both 
using the same internal bus?) or simplifying assumptions in the 
arbitrator implementation.

[snip]

Best regards,
gvb