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

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Date:	Wed, 14 Oct 2009 23:33:50 +0200
From:	Krzysztof Halasa <khc@...waw.pl>
To:	ddutile@...hat.com
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

Don Dutile <ddutile@...hat.com> writes:

> so, maybe the right terms are
>  2.5 GHz PCI-E
>  5.0 GHz 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).

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.

> So, the data rate is not stated, just the cycle rate.

Cycle rate, sure. Frequency, no.

> 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).
-- 
Krzysztof Halasa
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