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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

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.

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.