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

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Date:	Wed, 14 Oct 2009 15:49:38 -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:
> Stefan Assmann <sassmann@...hat.com> writes:
> 
>> IMHO this is rather confusing, as most people don't know what GT/s means.
> 
> It's trivial to look it up, isn't it?
> 
>> So I'd suggest the following change:
>>
>> --- a/drivers/pci/hotplug/pci_hotplug_core.c
>> +++ b/drivers/pci/hotplug/pci_hotplug_core.c
>> @@ -86,8 +86,8 @@ static char *pci_bus_speed_strings[] = {
>>  	"66 MHz PCIX 533",	/* 0x11 */
>>  	"100 MHz PCIX 533",	/* 0x12 */
>>  	"133 MHz PCIX 533",	/* 0x13 */
>> -	"2.5 GT/s PCI-E",	/* 0x14 */
>> -	"5.0 GT/s PCI-E",	/* 0x15 */
>> +	"2.5 Gbps PCI-E",	/* 0x14 */
>> +	"5.0 Gbps PCI-E",	/* 0x15 */
> 
> Isn't it like calling 100BASE-TX a 125 Mb/s? _That_ would be confusing.
> BTW PCI-E can be multi-lane so Mb/s (and even MB/s) don't make sense.
> I guess many people don't know what a MHz is either but we don't say
> 133 MHz = 133 Mbps.

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

No matter how many lanes, or how the data is sent (long or short bursts),
the frequency rate is a constant.

So, the data rate is not stated, just the cycle rate.
This would follow the PCIX syntax as well, which is
void of bandwidth illusions.

- Don

ps -- "GT/s" isn't even defined in the PCIe spec, just
      blindly introduced, and unjustified in its use.
      ..... too much marketing-speak in PCI SIG ...

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