SuperMicro X9SRL slot bifurcation

Notice: Page may contain affiliate links for which we may earn a small commission through services like Amazon Affiliates or Skimlinks.

nabsltd

Well-Known Member
Jan 26, 2022
402
270
63
With version 3.3 of the BIOS, the X9SRL supports bifurcation. But, it's not just bifurcation...it's actually re-arrangement of the PCIe lanes.

Every slot seems to have all PCIe lanes wired to it, even if the active slot width when the board was made (and bifurcation wasn't really a thought) was fewer lanes. In other words, some of the the x8 and x16 slots can be re-mapped to use the full x8 or x16 lanes, even though the default for the slot might have been x4 or x8.

On the board, slots 2-4 have x16 lanes grouped, slot 5 has x8 lanes, and slots 6-7 have x16 lanes grouped. You can re-arrange the lanes assigned to each slot in a wide variety of ways (see picture). The only things you can't do is set the x16 lanes to x4x4x4x4 (and it isn't really necessary, anyway), slot 3 can't be x8, and slots 2 and 3 can't be x4x4.

This makes the board amazingly versatile.

So, my question: do the X10, X11, and X12 motherboards with similar slot setups (6-7 slots, with a mix of x8 and x16 physical) support the same versatility of bifurcation in the BIOS?
 
  • Wow
Reactions: itronin

nabsltd

Well-Known Member
Jan 26, 2022
402
270
63
slots 4/6 are x8 in x16 physical
Yeah, I can read the manual (that has not been updated since about version 1 of the BIOS), too.

Version 3.3 of the BIOS allows you to set the x16 physical slots to x16 electrical, with the other slots in the grouping at x0. There is literally zero value in providing such a feature in the BIOS if it didn't actually work.

Despite what the diagram says, the BIOS groups slots 2-4 and slots 6-7 together, and I've changed the settings and the BIOS is right.
 

RolloZ170

Well-Known Member
Apr 24, 2016
5,306
1,603
113
Version 3.3 of the BIOS allows you to set the x16 physical slots to x16 electrical
that needs PCIe switches, but there are no.
the IOU port is 16 bit, lower 8 goes to one PCIe slot, higher 8 to other.
 
Last edited:

RolloZ170

Well-Known Member
Apr 24, 2016
5,306
1,603
113
the BIOS groups slots 2-4 and slots 6-7 together, and I've changed the settings and the BIOS is right.
they are connected to the same IOU2 port.
Slot 4 comes from 2AB, Slot 2 from 2CD,
if you bifurcate IOU2 to x16, slot 2 will not work because only 2A is checked for devices, slot 4 remains x8
bif. x8x8 = slot2 x8 - slot4 x8
bif. x4x4x4x4 = slot2 x4x4 - slot4 x4x4
bif. x4x4x8 = slot2 x4x4 - slot4 x8
bif. x8x4x4 = slot2 x8 - slot4 x4x4
 
  • Like
Reactions: nexox

nexox

Well-Known Member
May 3, 2023
647
254
63
There is literally zero value in providing such a feature in the BIOS if it didn't actually work.
Seems to me like they just half-assed the implementation without bothering to remove invalid options, it would be really cool if you could use these settings without referencing the block diagram from the manual, but that's kind of more than anyone expects on advanced BIOS config from Supermicro.
 

CyklonDX

Well-Known Member
Nov 8, 2022
817
265
63
Most if not all X11D* motherboards
Allows bifurcation on all pcie 3.0 x16 ports (x8x8, x4x4x4, x4x4x8, x8x4x4, auto)
(if intel vmd is disabled)

X11DPG-QT
You do not have daisy chaining so each x16 slot is literally up to you to decide.
1708479045924.png
 

nabsltd

Well-Known Member
Jan 26, 2022
402
270
63
So, it turns out that both of the following are true: the manual for the X9SRL is wrong and the BIOS has useless bifurcation settings.

The BIOS has the actual groupings of the slots correct (2-4, 5, 6-7) with (16, 8, 16) total lanes to assign to them. And, all the BIOS settings which actually look like they bifurcate (i.e., split a slot into x4x4) work as they should.

And, the settings that look like they move around lanes as if there was a PEX chip do reduce lane counts as they indicate (e.g., the x16 setting on lanes 2-4 gives all the available lanes to slot 4), but slots can't have more lanes than the default (x4 or x8, depending on the slot). Since there is another menu that can completely turn off any slot, any setting that decreases lanes on a slot to zero is useless.

The really weird thing is that on slots 2-4, the default "x8x4x4" results in 4:x8/3:x4/2:x4 while "x8x8" results in 4:x8/3:x0/2:x4 and "x4x4x8" results in 4:x4/3:0/2:x4. So, it's slot 3 that loses lanes, not slot 2.
 
Last edited:

nexox

Well-Known Member
May 3, 2023
647
254
63
Is it possible you got an odd variant of the X9SRL? I feel like I have seen one on eBay which had a silkscreen that indicated it could do x16/x8 + x0/x8 electrical on the two x16 slots, which requires a passive PCIe switch (where a typical PLX would be considered an active switch,) and they're usually operated based on which cards are installed, not with a BIOS setting (though I'm sure that exists.)

Still I haven't seen that kind of thing on slots smaller than x16.