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Bifurcation has never been a thing on desktop platforms and even most entry-level (single socket, desktop-equivalent) servers don't support it. It seems to be r

Bifurcation has never been a thing on desktop platforms and even most entry-level (single socket, desktop-equivalent) servers don't support it. It seems to be reserved for HEDT and real server platforms. (This is of course purely a market segmentation decision by Intel/AMD).

toast0 5y ago
My A520 mini-itx board supports it; can't get anymore desktop than that. Although, that has limited options, I think I can do either 2 x8 or one x8 and two x4. For this, it looks like each drive is expected to be x1, so you'd want one x16 to 16 x1s. It's doable, but not without mucking about in the firmware (either by the OEM, or dedicated enthusiasts), so a PCIe switch is probably advisable.
- formerly_proven 5y ago
  Ah I see. It seems like previously bifurcation was not qualified for anything but the X-series chipset, but in the 500 series it's qualified for all. On top of that, it seems like some boards just allowed it regardless in prior generations. Another complication is of course that the non-PEG slots on the (non-HEDT) platforms are usually electrically only x4 or x1, so bifurcation really only makes sense in the PEG.
  - wtallis 5y ago
    PCIe root ports in CPUs are generally designed to provide an x16 with bifurcation down to x4x4x4x4 (or merely x8x4x4 for Intel consumer CPUs). Large-scale PCIe switches also commonly support bifurcation only down to x4 or sometimes x2, though x1 may start catching on with PCIe gen5. Smaller PCIe switches and motherboard chipsets usually support link widths from x4 down to x1. Treating each lane individually goes hand in hand with the fact that many of the lanes provided by a motherboard chipset can be reconfigured between some combination of PCIe, SATA and USB: they design a multi-purpose PHY, put down one or two dozen copies of it at the perimeter of the die, and connect them to an appropriate variety of MACs.
    - formerly_proven 5y ago
      Yeah, but what I meant above was that if you only have an x4 slot electrically, sticking in an x16 -> 4x M.2 riser isn't going to do a whole lot, because the 12 lanes of 3 out of 4 slots aren't hooked up to anything. So in this scenario you'd really want a riser with a switch in it instead (which are more expensive than almost all motherboards). So on the consumer platforms that give you two PEGs best you could do while still having a GPU is stick that riser in the second PEG and use the x8/x8 split. Now the question becomes whether the UEFI allows you to use the x8/x8 bifurcation meant for dual GPU or similar use in an x8/(x4+x4) triple bifurcation kind of setup. Realistically this entire thing just doesn't make a lot of sense on the consumer platforms because they just don't have enough PCIe lanes out of the CPU. Intel used to be slightly worse here with 20 (of which 4 are reserved for the PCH but you know that), while AM4 has 28 (4 for the I/O hub again). On an HEDT platform with 40+ lanes though... (When I say bifurcation I meant bifurcation of a slot on the mainboard, not the various ways the slots and ports on the board itself can be configured, though that's technically bifurcation as well (or even switching protocols)
      - wtallis 5y ago
        > Yeah, but what I meant above was that if you only have an x4 slot electrically, sticking in an x16 -> 4x M.2 riser isn't going to do a whole lot, because the 12 lanes of 3 out of 4 slots aren't hooked up to anything. So in this scenario you'd really want a riser with a switch in it instead (which are more expensive than almost all motherboards). True; but given how PCIe speeds are no longer stalled, we may soon see motherboards offering an x4 slot that can be operated as x1x1x1x1. Currently, the only risers you're likely to find that split a slot into independent x1 ports are intended for crypto mining, and they require switches. A passive (or retimer-only) quad-M.2 riser that only provides one PCIe lane per drive currently sounds a bit bandwidth-starved and wouldn't work with current motherboards. But given PCIe gen5 SSDs or widespread availability of PCIe-native hard drives, those uses for an x4 slot will start to make sense.
wtallis 5y ago
PCIe bifurcation works fine on AMD consumer platforms. I've used a passive quad-M.2 riser card on an AMD B550 motherboard with no trouble other than changing a firmware setting to turn on bifurcation. It's only Intel that is strict about this aspect of product segmentation.
rasz 5y ago
Intel supported PCIE bifurcation it on desktop chipsets up to Sandy Bridge. Just like Intel once supported memory parity on desktop chipsets until deciding to use it to upsell server products. Recent Intel chipsets were locked to Intel SSDs and only some magic hardware DRM handshake unlocked bifurcation https://www.asus.com/us/support/FAQ/1037507/ https://www.asus.com/us/support/FAQ/1037507/ But apparently AMD competition forced their hand and at lest some got unlocked https://forum.level1techs.com/t/new-bios-update-unlocks-4x-pcie-x4-bifurcation-and-raid-on-existing-x299-motherboards/1510701 https://forum.level1techs.com/t/new-bios-update-unlocks-4x-p... On AMD front there doesnt seem to be any problems, plenty of desktop boards with unlocked bifurcation BIOS menus.

toast0 5y ago

My A520 mini-itx board supports it; can't get anymore desktop than that. Although, that has limited options, I think I can do either 2 x8 or one x8 and two x4. For this, it looks like each drive is expected to be x1, so you'd want one x16 to 16 x1s. It's doable, but not without mucking about in the firmware (either by the OEM, or dedicated enthusiasts), so a PCIe switch is probably advisable.

formerly_proven 5y ago
Ah I see. It seems like previously bifurcation was not qualified for anything but the X-series chipset, but in the 500 series it's qualified for all. On top of that, it seems like some boards just allowed it regardless in prior generations. Another complication is of course that the non-PEG slots on the (non-HEDT) platforms are usually electrically only x4 or x1, so bifurcation really only makes sense in the PEG.
- wtallis 5y ago
  PCIe root ports in CPUs are generally designed to provide an x16 with bifurcation down to x4x4x4x4 (or merely x8x4x4 for Intel consumer CPUs). Large-scale PCIe switches also commonly support bifurcation only down to x4 or sometimes x2, though x1 may start catching on with PCIe gen5. Smaller PCIe switches and motherboard chipsets usually support link widths from x4 down to x1. Treating each lane individually goes hand in hand with the fact that many of the lanes provided by a motherboard chipset can be reconfigured between some combination of PCIe, SATA and USB: they design a multi-purpose PHY, put down one or two dozen copies of it at the perimeter of the die, and connect them to an appropriate variety of MACs.
  - formerly_proven 5y ago
    Yeah, but what I meant above was that if you only have an x4 slot electrically, sticking in an x16 -> 4x M.2 riser isn't going to do a whole lot, because the 12 lanes of 3 out of 4 slots aren't hooked up to anything. So in this scenario you'd really want a riser with a switch in it instead (which are more expensive than almost all motherboards). So on the consumer platforms that give you two PEGs best you could do while still having a GPU is stick that riser in the second PEG and use the x8/x8 split. Now the question becomes whether the UEFI allows you to use the x8/x8 bifurcation meant for dual GPU or similar use in an x8/(x4+x4) triple bifurcation kind of setup. Realistically this entire thing just doesn't make a lot of sense on the consumer platforms because they just don't have enough PCIe lanes out of the CPU. Intel used to be slightly worse here with 20 (of which 4 are reserved for the PCH but you know that), while AM4 has 28 (4 for the I/O hub again). On an HEDT platform with 40+ lanes though... (When I say bifurcation I meant bifurcation of a slot on the mainboard, not the various ways the slots and ports on the board itself can be configured, though that's technically bifurcation as well (or even switching protocols)
    - wtallis 5y ago
      > Yeah, but what I meant above was that if you only have an x4 slot electrically, sticking in an x16 -> 4x M.2 riser isn't going to do a whole lot, because the 12 lanes of 3 out of 4 slots aren't hooked up to anything. So in this scenario you'd really want a riser with a switch in it instead (which are more expensive than almost all motherboards). True; but given how PCIe speeds are no longer stalled, we may soon see motherboards offering an x4 slot that can be operated as x1x1x1x1. Currently, the only risers you're likely to find that split a slot into independent x1 ports are intended for crypto mining, and they require switches. A passive (or retimer-only) quad-M.2 riser that only provides one PCIe lane per drive currently sounds a bit bandwidth-starved and wouldn't work with current motherboards. But given PCIe gen5 SSDs or widespread availability of PCIe-native hard drives, those uses for an x4 slot will start to make sense.

wtallis 5y ago

PCIe bifurcation works fine on AMD consumer platforms. I've used a passive quad-M.2 riser card on an AMD B550 motherboard with no trouble other than changing a firmware setting to turn on bifurcation. It's only Intel that is strict about this aspect of product segmentation.

rasz 5y ago

Intel supported PCIE bifurcation it on desktop chipsets up to Sandy Bridge. Just like Intel once supported memory parity on desktop chipsets until deciding to use it to upsell server products. Recent Intel chipsets were locked to Intel SSDs and only some magic hardware DRM handshake unlocked bifurcation https://www.asus.com/us/support/FAQ/1037507/ https://www.asus.com/us/support/FAQ/1037507/ But apparently AMD competition forced their hand and at lest some got unlocked https://forum.level1techs.com/t/new-bios-update-unlocks-4x-pcie-x4-bifurcation-and-raid-on-existing-x299-motherboards/1510701 https://forum.level1techs.com/t/new-bios-update-unlocks-4x-p... On AMD front there doesnt seem to be any problems, plenty of desktop boards with unlocked bifurcation BIOS menus.