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Date: Sun, 18 Sep 2011 23:06:47 +0200
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: Arnaud Lacombe <lacombar@gmail.com>
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Cc: jfv@freebsd.org, pyunyh@gmail.com, Hooman Fazaeli <fazaeli@sepehrs.com>,
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Subject: Re: intel checksum offload
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On Sun, Sep 18, 2011 at 03:19:46PM -0400, Arnaud Lacombe wrote:
> Hi,
> 
> On Sat, Sep 17, 2011 at 4:32 PM, YongHyeon PYUN <pyunyh@gmail.com> wrote:
> > On Sat, Sep 17, 2011 at 11:57:10AM +0430, Hooman Fazaeli wrote:
> >> Hi list,
> >>
> >> The data sheet for intel 82576 advertises IP TX/RX checksum offload
> >> but the driver does not set CSUM_IP in ifp->if_hwassist. Does this mean that
> >> driver (and chip) do not support IP TX checksum offload or the support for
> >> TX is not yet included in the driver?
...
> This is slightly off-topic, but still..
> 
> FWIW, I'm not really impressed by what chips claim to support vs. what
> has been implemented in the driver. As per the product brief, the
...
> [0]: the commit message say "performance was not good", but it is not
> the driver's developer to decide whether or not a feature is good or
> not. The developer's job is to implement the chip capabilities, and
> let it to the user to enable or disable the capabilities. At best, the
> developer can decide whether or not to enable the feature by default.

actually, this is a perfect example where the developer has done the
right thing: implemented the feature, verified that performance is bad,
hence presumably removed support for the feature from the code (which also
means that the normal code path will run faster because there are no
run-time decisions to be made).

"optional" features are often costly even when disabled.

cheers
luigi