Ethernet problem again!
When I used ethtool to change the net speed from 10M to 100M, the PHY state changed to unlink state and never goes back to link sate.
Below is the result after some tests.
1. when in unlink state, switch to aoto-negotiation will fix this problem, phy state change to link again
2. can not fix this problem by down/up eth0
3. change speed from 100M to 10M is fine
4. when I said "never back to link state", it is a little mistake, sometimes it did go back to link state after a long long long time.
I try to fix it by the patch at this link, but it won't help.
PS: I fixed the "swinging" problem by used the new LAN8720 driver(from kernel 4.9.9, drivers/net/phy/smsc.c, the kernel I used now is 4.1.15)