RFC: Evolution of the em driver



I have an important decision to make and I thought rather than just make
it and spring it on you I'd present the issues and see what opinions were.

Our newer hardware uses new features that, more and more, require
parallel code paths in the driver. For instance, the 82575 (Zoar) uses
what are called 'advanced descriptors', this means different TX path.
The 7.0 em driver has this support in it, it just uses a function pointer
to handle it.

When I add in multiqueue/RSS support it will add even more code
that functions this way.

What the Linux team did was to split the newer code into a standalone
driver, they call it 'igb'. I had originally resisted doing this, but with
the development I have been working on the past month I am starting
to wonder if it might not be best to follow them.

I see 3 possibilities and I'd like feedback, which would you prefer if
you have a preference and why.

First, keep the driver as is and just live with multiple code paths
and features, possibly #ifdef'ed as they appear.

Second, split the driver as Linux has into em and igb. The added
question then is how to split it, Linux made the line the use of
advanced descriptors, so Zoar and after, but I could also see a
case for having everything PCI-E/MSI capable being in the new
driver.

Third, sort of a half-way approach, split up code but not the
driver, in other words offer different source files that can be
compiled into the driver, so you could have the one big jumbo
driver with all in there, or one that will only work with a subset
of adapters. This one would probably be the most work, because
its a new approach.

Cheers,

Jack
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