Rick Jones wrote:
With mii-tool we can do the command below and work with a half duplex
hub and a full duplex switch.
mii-tool -A 10baseT-FD,10baseT-HD eth0
Why, and how often, is that really necessary?
rick jones
Take a wifi router with only 802.11b radios and ethernet chips that do
not support handling DMA to host memory. In this case receives from
ethernet cost a lot since the packets must be read into host memory by
the CPU (instead of written there by the adapter). Also, receiving 100
Mbit in when you only have 802.11b pipes out isn't terrible useful. UDP
blasts from ethernet can hog the CPU and with some NICs cause soft lockups.
We could just set it to 10Mbit half duplex always but two 802.11b radios
can saturate that with bidirectional traffic. We have seen failures to
send due to collisions. Thus, we would like to be able advertise 10Mb
full and half duplex so that we can get the appropriate speed when that
the neighbor device supports. We have seen various ethernet switches or
NICs (like IP video cameras with ethernet) that will not run 10Mb full
duplex but will do 100Mb full or 10Mb half.
We want it to be configurable at run time since sometimes the same
software may run on a system that has 802.11G radios and and perhaps
more capable ethernet devices. We can detect our situation in our
startup script and adjust the advertised speeds down only when needed.
-Ack
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