Subject: Re: TSO on wm(4) (Intel Pro/1000): i82546 vs i8254EI vs others?
To: Jonathan Stone <email@example.com>
From: Jason Thorpe <firstname.lastname@example.org>
Date: 05/23/2005 20:37:44
On May 23, 2005, at 8:00 PM, Jonathan Stone wrote:
> Yamaoto-san, that strikes me as sheer nonsense. In Jason's reply to
> my original message, Jason reports that *Intel* says that, on this
> specific chip, TSO doesn't work as specified. Intel says it doesn't
> work. Why, then, is wm(4) advertising the capability? Is it just
> "because Linux does?", or because Matt Thomas wasn't aware of
> that particular errata?
Empirically, it "works". That is to say it exercises the offload
code path as intended and generates valid packets on the wire.
> No, that doesn't match the facts in this case. Jason reports that
> Intel issued an errata saying that *on this specific chip*, the 82544,
> TSO (aka LSO) doesn't work to spec. At all. It's busted. My findings
> agree with Jason's report.
And that could simply have been "didn't meet product marketing's
performance requirements". The 82547 is way more broken than the
> Remember: the current state of affairs is that an i82544 is attached
> with CSUM_TSOv4 advertized. But when enabled, the TSO offload doesn't
> work to spec.
> I prefer to emit a one-line warning when 82544 devices are attached,
> saying that TSOv4 is enabled but adoesn't work properly. Once warned,
> a careful admin may then check the manual or the source code.
But from an end-user's perspective, it DOES work "properly". It just
doesn't provide much benefit on that particular chip (but even your
measurements indicate that there is _some_ benefit).