How to clear WC buffer, or is it needs to clear WC buffer

[april]

 Hi all:

 I have a problem. I want to make a command buffer in PCIE memory, and
 make Engine execute those commands.
 First set PCIE memory to be WC, copy command to command buffer in PCIE
 memory, and make HW to execute those commands.
 sometimes, the Graphic engine will hang.

 I try to use mb() to clear WC buffer, it seems a little butter?  But
 the problem is still.

 When I set the PCIE memory to Uncached, everthing is OK.

 So, Is it due to the write buffer that is not evicted ?

 and how to clear the WC buffer?

 Thanks