TFTP RFC7440 windowsize support

[Sascha Hauer]

Hi All,

As you might have noticed Enrico added RFC7440 support to our TFTP
support. This greatly improves the speed for TFTP transfers when the
server also has RFC7440 support.

RFC7440 support has just appeared in the latest atftpd release
(version 0.8.0), so you can give this a try without locally patched
TFTP demons. This release is also available in debian backports.

Unfortunately there's no light without shadow. On i.MX6 the FEC ethernet
controller is too slow to catch up with a gigabit link. Up to now this
hasn't been a problem on barebox, as we only ever had a single packet
in flight. Now with windowsize support the controller can get
overwhelmed with so many TFTP packets that RX overruns occur. I changed
the imx_v7_defconfig to limit the windowsize to a smaller value that
seems to work here. Similar problems might exist on other ethernet
controllers, so we might have to generally switch to more conservative
default window sizes.

Sascha

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