The problem with using stmmac as an example is that it is very old. It
was added to Linux in 2009. I guess most of the SoCs at that time were
single or dual core. CPU affinity was not something developers thought
about for that class of SoC.
Over time the number of cores has gone up and stmmac has got faster
link speeds. But is the architecture correct? Is the driver following
best practices?
I would suggest you look at more modern MAC drivers and see how they
do CPU affinity, etc. Is there anything which can be learned from them
and implemented in stmmac?
Andrew