TODO:
	to make DMA mode work
	test spidev3.0 & spidev3.1
	trial: how to make a kernel module to enable spidev?

test the max speed: 
	speed=20M, real speed=12MHz; 
	speed=30M, real speed=24MHz;
	speed=48M, real speed=48MHz, and this is the max value
	
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For kernel version 3.8.11-x3.4, after reboot, no /dev/spidev cannot be found.
While for kernel version 3.0.4-x3, after reboot:
ubuntu@omap:~$ ls /dev/spidev* -al
crw------- 1 root root 153, 0 2013-07-26 06:45 /dev/spidev3.0
crw------- 1 root root 153, 1 2013-07-26 06:45 /dev/spidev3.1
crw------- 1 root root 153, 2 2013-07-26 06:45 /dev/spidev4.0

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One way is below:
ubuntu@omap:~$ cat /proc/devices | grep spi
153 spi

And then mknod /dev/spidev4.0 c 153 0
But it doesn't work well, too!
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Solution from RobertCNelson:

add "buddy=spidev" to your bootargs, such that the kernel will enable spidev... 

Tom Xue: that is to add "buddy=spidev" to uEnv.txt, which is in boot partition of SD card
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