[lwip-devel] [bug #20409] Add netif callback function for link change event

[Jared Grubb]

Follow-up Comment #10, bug #20409 (project lwip):

> when the DHCP client is negociated an ip address, the netif is "down"

I haven't used the DHCP module at all. Now I understand what you mean. So,
instead of 4 states, there are 6. Let's agree on the states the application,
stack, and driver should understand (correct them if wrong, I'm just doing
this off the top of my head):

UP = application is allowed to use this interface
DOWN = application is not allowed to use this interface

LINK = driver is able to RX and TX
NO LINK = driver cannot RX and TX

STARTED = driver is active and running
STOPPED = driver has been initialized, but is inactive

In this definition, there would be the following:
(DOWN,NO LINK,STOPPED) All off, driver circuits maybe powered down
(DOWN,NO LINK,STARTED) Driver running but no link found (yet)
(DOWN, LINK  ,STARTED) Driver ready to go. DHCP would neogtiate here.
( UP , LINK  ,STARTED) All good.
( UP ,NO LINK,STARTED) Lost link. Driver returns error on transmit. Perhaps
DHCP would bring down?

The following states would not be allowed:
(DOWN, LINK  ,STOPPED) Stopped implies no link
( UP , LINK  ,STOPPED) UP & STOPPED is invalid
( UP ,NO LINK,STOPPED) UP & STOPPED is invalid

What do you think of this much?

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