The maxfrequency of gpio of AM3354

____-__-__ said:

 However, the frequency of the signal present at gpio pin can only achive almost 3MHz, i don't know why? Is the gpio have the ability to achive a frequency as higher as 20MHz?

That's not terribly surprising.  I expect you fall into one of these two cases:

1. MMU configured, and all register address ranges mapped as "strongly ordered".   In this case, you since you are performing only writes you could likely see a huge increase by changing the mapping to "device" which would allow buffered writes.
2. MMU not configured, in which case everything defaults to "strongly ordered".

In general there are a lot of things that prevent max performance: MMU configuration, CPU pipelining, interconnect latency, crossing clock domains, etc.  The BEST solution would be to use the PRU subsystem.  It was created to solve this precise problem.  Specifically it includes local, deterministic I/O capabilities.  More info can be found here:

http://processors.wiki.ti.com/index.php/PRU_Training:_Hands-on_Labs

You may also want to check out this article:

http://processors.wiki.ti.com/index.php/Common_Issue_Resulting_in_Slow_External_Memory_Performance

There's a lot of overlap between the discussion and that article and the performance issue you're observing here.

Brad