igb and ALTQ in 9.1-rc3

[Barney Cordoba]

--- On Tue, 12/11/12, Gleb Smirnoff <glebius at FreeBSD.org> wrote:

> From: Gleb Smirnoff <glebius at FreeBSD.org>
> Subject: Re: igb and ALTQ in 9.1-rc3
> To: "Jack Vogel" <jfvogel at gmail.com>
> Cc: "Clement Hermann (nodens)" <nodens2099 at gmail.com>, "Barney Cordoba" <barney_cordoba at yahoo.com>, freebsd-net at FreeBSD.org
> Date: Tuesday, December 11, 2012, 2:58 AM
> On Mon, Dec 10, 2012 at 03:31:19PM
> -0800, Jack Vogel wrote:
> J> UH, maybe asking the owner of the driver would help
> :)
> J> 
> J> ... and no, I've never been aware of doing anything to
> stop supporting altq
> J> so you wouldn't see any commits. If there's something
> in the altq code or
> J> support (which I have nothing to do with) that caused
> this no-one informed
> J> me.
> 
> Switching from if_start to if_transmit effectively disables
> ALTQ support.
> 
> AFAIR, there is some magic implemented in other drivers that
> makes them
> modern (that means using if_transmit), but still capable to
> switch to queueing
> mode if SIOCADDALTQ was casted upon them.
> 

It seems pretty difficult to say that something is compatible with
something else if it hasn't been tested in a few years.

It seems to me that ATLQ is the one that should handle if_transmit. 
although it's a good argument for having a raw "send" function in 
drivers. Ethernet drivers don't need more than a send() routing that
loads a packet into the ring. The decision on what to do if you can't 
queue a packet should be in the  network layer, if we must still call
things layers. 

"start" is a leftover from a day when you stuffed a buffer and waited
for an interrupt to stuff in another. The whole idea is antiquated.

Imagine drivers that pull packets off of a card and simply queue it;
and that you simply submit a packet to be queued for transmit. Instead
of trying to find 35 programmers that understand all of the lock BS,
you only need to have a couple.

I always disable all of the gobbledegook like checksum offloading. They
just muddy the water and have very little effect on performance. A modern
cpu can do a checksum as fast as you can manage the "capabilities" without
disrupting the processing path. 

With FreeBSD, every driver is an experience. Some suck so bad that they
should come with a warning. The MSK driver is completely useless, as
an example.


BC