Vc initiation process:
（1）FC_INIT1
（2）FC_INIT2
While in FC_INIT1, block tx tlp using VCx
Non-flit mode
(1)FC1-P (2)FC1-NP (3)FC1-CPL -> all dedicated
Flit mode
CREDITS_CONSUMED
Count of the total number of FC units consumed by TLP Transmissions made since Flow Control initialization
CREDIT_LIMIT****is fromFC updatedllp
reflects all credit flow control updates, for each FC update received, if CREDIT_LIMIT is not equal to the update value, set CREDIT_LIMIT to the update value
Update_FC **内的Credit值是Rx FC Buffer初始Credit及所有从该Buffer取出的TLP数据占用Credit的总和，是累加的。**想要获知 Rx FC Buffer 余量，对于 Rx 端需要综合 CA 及 CR 两个计数器，对于 Tx 端则是 CL 及 CC 两个计数器
对于TX来说是CL-CC；
对于RX来说是CA-CR
CREDITS_ALLOCATED, Count of the total number of credits granted to the Transmitter since initialization
Rx一共允许tx发送多少内容,rx处理完毕相应的事务之后这个cnt增加，递增
CREDITS_RECEIVED, Count of the total number of FC units consumed by valid TLPs Received since Flow Control initialization
已经接收的数量
TLP进入Rx FC Buffer 之前，Receiver可判断 Rx FC Buffer是否有能力接收。若以下条件成立，表明 Rx FC Buffer 会溢出
