OT: new Gigabyte motherboard weirdness

[Rajinder Yadav]

Lennart Sorensen wrote:
> 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.
> 
ok like I just learned something new =) thanks!
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