linux-kernel - Re: GT/s vs Gbps for PCIe bus speed

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Message-ID: <4AD655E1.7080005@redhat.com>
Date:	Wed, 14 Oct 2009 18:51:13 -0400
From:	Don Dutile <ddutile@...hat.com>
To:	Krzysztof Halasa <khc@...waw.pl>
CC:	Stefan Assmann <sassmann@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	kaneshige.kenji@...fujitsu.com, matthew@....cx
Subject: Re: GT/s vs Gbps for PCIe bus speed

Krzysztof Halasa wrote:
> Don Dutile <ddutile@...hat.com> writes:
> 
>> so, maybe the right terms are
>>  2.5 GHz PCI-E
>>  5.0 GHz PCI-E
> 
Yeah, the std nomenclature is PCI-e not PCI-E.

> I don't thinks so. It would be fine for PCI/PCI-X, as there is a clock
> signal with a given frequency. PCI-E doesn't use a clock signal. Really,
> the meaningful value is a cycle time (or number of cycles per second).
> 
number of cycles/second == frequency.
cycle time = 1/frequency

>From a run-time perspective, the status is trying to
tell the user/admin what (steady-state) frequency the
links are running at : 2.5GHz or 5.0GHz.

> Of course one could calculate or measure a frequency (or spectrum) for
> a given code sequence on PCI-E. For example, for something like
> 01010101010101 (raw code) the (base) frequency would be 1.25 or 2.5 GHz
> for 2.0. For other patterns it would be lower.
> 
>> No matter how many lanes, or how the data is sent (long or short bursts),
>> the frequency rate is a constant.
> 
> Actually, this is not the case.
> 
Frequency changing would require link re-synch.
This code is dealing w/steady-state frequency.

>> So, the data rate is not stated, just the cycle rate.
> 
> Cycle rate, sure. Frequency, no.
> 
I think nomeclature is mixed up here.

>> This would follow the PCIX syntax as well, which is
>> void of bandwidth illusions.
> 
> Bandwidth, actually it may make some sense. But it would have to take
> #lanes into account, I'm not sure we want to do it. And it would create
> another confusion - raw vs effective bandwidth (like 125 vs 100 Mbps
> with Ethernet).

Again, trying to generate output that relates
to what devices are spec to run at: 2.5GHz or 5.0GHz links.

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