PCIE LANE翻转和PN翻转（lane reversal and polarity）

为了布线方便，PCIE支持lane翻转和PN翻转，但是必须遵守以下原则
1，PN翻转没有限制，可以翻转部分lane，也可以只翻转TX或RX
PN翻转是接收端的特性，接收端检测到PN翻转后会对接收到的数据进行PN翻转。
Polarity inversion is a function of the receiver and not the transmitter. The trans-mitter never inverts its data. During link training, the receiver examines symbols 6 through 15 of the TS1 and TS2 ordered sets for inversion of the PExRP[n] and PExRN[n] signals. If an inversion is detected, then logic for the receiving lane automatically inverts received data. Polarity inversion is a lane function and not a link function. Therefore, it is possible for some lanes of link to be inverted and for others not to be
2，lane 翻转必须全部翻转，不能部分翻转
3，lane翻转TX/RX必须同时翻转，不能只翻转TX或RX
4，lane翻转和PN翻转可以同时存在