How to control am3517 gpio output high and low level?

I just figured this out myself.  The first thing to do was to ensure that the pin I wanted was set to a GPIO.  Since I'm using the AM3517 evm I edited the u-boot file for the pin mux settings (AM3517evm.h).  You could also use the Pin Mux Utility.  

Then I rebuilt and reloaded U-boot, and rebuilt and reloaded the kernel.

Then I used the steps to access the GPIO through sysfs as described in this post:

http://e2e.ti.com/support/dsp/sitara_arm174_microprocessors/f/416/p/106693/376227.aspx#376227

To control the GPIO from within a simple C program, I followed what was written here:

http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:gpio-sysfs

There is a similar file from TI in the PSP (linux-03.00.00.05/drivers/gpio/gpiolib.c), but this one was easier for me to understand as it was more basic.

I don't know if you want to control GPIO from the userspace, but it worked for my test at least.  Hope that helps.