Question Caveats of using Ryzen 4750G with MSI B550 UNIFY / Gigabyte B550 Arous Master

[Maheshpm]

Hello,

MSI B550 UNIFY Board has total four M2 slots. 1st is directly coupled with CPU, 2nd and 3rd M2 slots actually share PCI-E 4.0 x 16 bandwidth. If I put SSD in any of these M2 slots, PCI-E slot brought down to x8 mode. This is fine as long as there is 3000/5000 series CPU with graphics card installed in PCI-E x 16 slot.

I just wanted to couple Ryzen 4750G (processor with CPU graphics) with MSI B550 UNIFY and will not using any discrete graphics card. Will this disable two middle M2 slots ? Because there are total 16 lanes available for CPU built-in graphics and i am not sure if built-in VEGA CPU graphics will run with x8 and allows SSDs to run in both M2 slots with x4 and x4 mode coupled to CPU?
Applicable for B550 UNIFY and B550 Arous Master

From MSI board manual:

In CPU mode with AMD RyzenTM 4000-G series processors, if you install the MSI M.2 Xpander series add-in card into PCI_E1 slot, only one M.2 slot of the add-in card is available.

In above PCI_E1 refers to full length PCI-E 4.0 x 16 slot next to CPU

I will not use any M2 card in PCI-E slot but based on above statement from manual, after using CPU with IGPU, I am left with very few lanes with main PCI-E 4.0 x 16 slot. It means somewhere IGPU consumes available PCI lanes.

There are total 16 PCI lanes available for this board connected to CPU (PCI-E 4.0 x 16) as per B550 specifications. After using CPU with IGPU if those lanes brought down to extent that i can use only single M2 slot from add-in card in main PCI-E 4.0 x 16 slot (I am not adding M2 card) , then I don't think it allows me to use onboard M2 slot as they actually shares bandwidth with main PCI-E 4.0 x 16 slot

Any pointers would be highly appreciated

 

[damian101]

Why would a PCIe slot using CPU pcie lanes be brought down to x8 mode when an SSD is put into a slot connected to chipset PCIe slots? That doesn't make much sense to me. Switching between different PCIe lane sources sounds way too complicated for a normal consumer board to me.
For the same reason I doubt that slots connected to chipset PCIe lanes would suddenly be connected to CPU PCIe lanes if you are using the board with a Ryzen APU instead of a standard Ryzen CPU.

In CPU mode with AMD RyzenTM 4000-G series processors, if you install the MSI M.2 Xpander series add-in card into PCI_E1 slot, only one M.2 slot of the add-in card is available.

I think that just means that bifurcation is not supported.

 

[Maheshpm]

For the same reason I doubt that slots connected to chipset PCIe lanes would suddenly be connected to CPU PCIe lanes if you are using the board with a Ryzen APU instead of a standard Ryzen CPU.

2nd and 3rd M2 slot by default connected to Chipset, however that behavior can be changed from BIOS so that main PCI-E slot gets converted to 8x and allows M2 slots to connected to CPU by 4x mode. That's true in case standard Ryzen CPU.

What am not getting is that how we can brought those 16 pci lanes (8x for two M2 and 8x for PCI-E slot) since CPU graphics demand is already completed with APU and it has actually no reason to look at those slots - in that case slots would get connected to chipset and no modifications would be possible?

if built in Graphics uses 0 PCI-E Lanes, then only it can utilize PCI lanes with m2 slots..I am not sure about this....

 

[Maheshpm]

any response please?