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

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Date:	Wed, 14 Oct 2009 10:42:22 +0200
From:	Stefan Assmann <sassmann@...hat.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
CC:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	kaneshige.kenji@...fujitsu.com, matthew@....cx, ddutile@...hat.com
Subject: GT/s vs Gbps for PCIe bus speed

Hi all,

I'm really confused about GT/s¹ vs Gbps². The PCIe 2.0 Base Spec (rev 0.9)
on page 34, section 1.2 speaks of "Signaling rate - [...] For the first
generation of PCI Express technology, there is only one signaling rate
defined, which provides an effective 2.5 Gigabits/second/Lane/direction
of raw bandwidth." but later in the document it purely speaks of 2.5
GT/s.

If I understand this correctly it means the following:
PCIe has a raw bandwidth of 2.5 Gbps (or 5.0 Gbps, whatever) but because
of the "8b/10b" encoding the effective bit rate is only
2.5 Gbps * (8/10). So it's called 2.5 GT/s to explicitly say this is raw
bandwidth.

IMHO this is rather confusing, as most people don't know what GT/s means.

So I'd suggest the following change:

diff --git a/drivers/pci/hotplug/pci_hotplug_core.c b/drivers/pci/hotplug/pci_hotplug_core.c
index 0325d98..75ef3d7 100644
--- 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 */
 };

 #ifdef CONFIG_HOTPLUG_PCI_CPCI

¹ Gigatransfers/second
² Gigabits/second

  Stefan
--
Stefan Assmann         | Red Hat GmbH
Software Engineer      | Otto-Hahn-Strasse 20, 85609 Dornach
                       | HR: Amtsgericht Muenchen HRB 153243
                       | GF: Brendan Lane, Charlie Peters,
sassmann at redhat.com |     Michael Cunningham, Charles Cachera
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