[U-Boot] netconsole: USB Ethernet connection dropping with ping or tftpboot

[Joe Hershberger]

On Tue, Feb 3, 2015 at 3:44 PM, Jörg Krause <jkrause at posteo.de> wrote:
> But if I use 'ping 10.0.0.1' or 'tftpboot u-boot.sb' the network
> connection drops. Both commands work fine if I switch back from
> netconsole to serial in-/output.
>
> This is the output from dmesg:
>         [31620.215354] usb 3-13: USB disconnect, device number 24
>         [31620.215422] cdc_ether 3-13:1.0 enp0s20u13: unregister
>         'cdc_ether' usb-0000:00:14.0-13, CDC Ethernet Device

One thing to note is that the network stack generally is designed to sit in
a state of inactive (i.e. devices halted / inactive). When a network
command is issued, the driver is initialized, then the command runs, then
the driver is halted again.

NetConsole breaks this assumption, since the network stack needs to be up
the whole time it is selected.  Net console used to bring the network
driver up and down for every character it sent.  Naturally this was a huge
problem on USB network adapters that don't come up fast or any other that
doesn't.  As a workaround (and its current state) when net console is used,
the network stack is lied to about the state of the driver (telling it the
driver is halted or inited) when the current and previous packets are net
console packets.  When a different type (ping or tftp, etc) of network
packet is sent, the driver is actually brought down and back up to ensure
the assumptions about the network stack hold true.  With more work we can
potentially make these better when net console is enabled.

Cheers,
-Joe