OT: new Gigabyte motherboard weirdness

[Lennart Sorensen]

On Fri, Jan 08, 2010 at 05:48:23PM -0500, Giles Orr wrote:
> Newegg is advertising a special on the new Gigabyte GA-P55A-UD3P
> motherboard.  I thought it looked nice, but wanted to read up on the
> specs.  Over at Gigabyte's site [1] I find out:
> 
> "When set Turbo SATA3 / USB3.0 (Marvell 9128 /NEC USB 3.0 Controller)
> to enable in BIOS setup, 1st PCIex16 slot will run at x8 bandwidth and
> 2nd PCIex16 slot will be disabled."
> 
> Why would you put a second PCI-e slot on the board if you're going to
> disable it when the user actually tries to _use_ other features?  The
> people who are likely to use both PCI-e slots are also the early
> adopters who are going to use SATA3 or USB3.  This strikes me as a
> poor design and a hell of a way to alienate people who didn't read
> their specs carefully enough.  I thought Gigabyte was better than
> this?
> 
> The 2nd PCIex16 slot was never actually x16: it runs at x4 at the best
> of times, so I guess the "x16" part only indicates the physical size
> of the slot.  As a non-gamer who runs Linux and wants multiple heads,
> was I likely to notice that my nvidia card in that slot was
> bandwidth-choked?  (I'm curious about the answer to this question
> without regard to the motherboard.)
> 
> 
> [1] http://www.gigabyte-usa.com/Products/Motherboard/Products_Spec.aspx?ClassValue=Motherboard&ProductID=3253&ProductName=GA-P55A-UD3P

The P55 systems (Core i5 LGA1156) only has a 16x PCIe 2.0 on the CPU and
a 4x PCIe 2.0 (called DMI) on the CPU (this one connects to the PCH chip
which has eight 1x PCIe 2.0 connections for add ons).

This means you can have one 16x video card slot, or on some CPUs you
can have two 8x video card slots.  Any other IO is at most 4x from the
PCH, so you can have one 4x slot, and a few 1x slots.

So if you have a USB 3.0 controller that needs 5Gbps, well that's more
than a 1x PCIe 2.0 link can carry (500MB/s so 4Gbps), so you need a 4x
link, which means you just took out the 4x video slot to enable the USB
3.0 controller.

The 6Gbps SATA controller with two connectors can use 12Gbps, so that
needs another 4x link minimum, so most likely it uses half the 16x
video slot (that is an 8x link) leaving the other 8xlink for the primary
video slot.

As for the people that want USB3.0 and 6Gbps SATA buying this... No
not really.  Those people would buy a Core i7 with the X58 chipset which
has plenty of PCIe links to handle this kind of load and still run two
video cards at 16x.  I don't know why they would put these new features
on a midrange platform.  P55 systems are not high end systems and were
never intended to be.

Now the note on the gigabyte web page doesn't mention the second video
slot going away, it says the first video slot goes from 16x to 4x when
you enable the SATA 6Gbps and USB 3.0 controllers, which sounds more
like they split the 16x into 8x for video and 4x for each controller.
This would certainly be better than starving the bandwith to the PCH
which already has 12 USB 2.0 ports, 6 SATA 3Gbps links and lots of other
stuff to do.

-- 
Len Sorensen
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