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List:       gentoo-user
Subject:    Re: [gentoo-user] Computer case for new build
From:       Frank Steinmetzger <Warp_7 () gmx ! de>
Date:       2023-09-19 17:05:12
Message-ID: ZQnUyCKqpv93Etgy () kern
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Am Tue, Sep 19, 2023 at 11:01:48AM -0400 schrieb Rich Freeman:

> > > The higher-end motherboards have switches, and not all
> > > the lanes may be the highest supported generation, but I don't think
> > > any modern AMD motherboards have any kind of PCIe controller on them.
> >
> > Here are the I/O capabilities of the socket:
> > https://www.reddit.com/r/Amd/comments/bus60i/amd_x570_detailed_block_diagram_pcie_lanes_and_io/
> 
> So, that is AM4, not AM5

Yup. I kept on rambling about AM4, because that's what I laid my eyes on 
(and so did Dale a few posts up).

> > A slight problem is that it is connected to the CPU by only 4.0×4. So tough
> > luck if you want to do parallel high-speed stuff with two PCIe×4 M.2 drives.
> 
> So, that block diagram is a bit weak.  If you look on the left side it
> clearly shows 20 PCIe lanes, and the GPU only needs 16.  So there are
> 8 lanes for the MB chipset to use.

No, the chipset downlink is always four lanes wide. PCIe 4.0 for most 
AM4 CPUs, but PCIe 3.0 for the monolithic APUs (because they don't have 
4.0 at all, as their I/O die is different). The remaining four lanes are 
reserved for an NVMe slot.

> The 4 on the left aren't the same as the 4 on the right I think.

The diagram is indeed a bit confused in that part.

> Again, that is AM4 which I haven't looked into as much.  AM5 increases
> the v5 lanes and still has some v4 lanes.

AFAIR, PCIe 5 is only guaranteed for the NVMe slot. The rest is optional or 
subject to the chipset. As in the A series doesn't have it, stuff like that. 
But it's been a while since I read about that, so my memory is hazy.

> All the same desktop CPUs are a bit starved for lanes.

Hey we did get four more now with AM5 vs. AM4.

> I'm sure PCIe v5 switching is hard/expensive, but they definitely
> could mix things up however they want.  The reality is that most IO
> devices aren't going to be busy all the time, so you definitely could
> split 8 lanes up 64 ways, especially if you drop a generation or two
> along the way.

Unfortunately you can't put low-speed connectors on a marketing sheet, when 
competitors have teh shizz.

> Server hardware definitely avoids many of the limitations, but it just
> tends to be super-expensive.

Which is funny because with the global cloud trend, you would think that its 
supply increases and prices go down.

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