Date: Fri, 11 May 2018 14:11:22 -0600 From: Warner Losh <imp@bsdimp.com> To: Wojciech Puchar <wojtek@puchar.net> Cc: Dieter BSD <dieterbsd@gmail.com>, "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>, freebsd-hardware@freebsd.org Subject: Re: PCIe multipliers, how do they work? Message-ID: <CANCZdfozrb5iacqNcu7werr8E=6tZr_WpcR_anq6Aw-xr1Xiyg@mail.gmail.com> In-Reply-To: <alpine.BSF.2.20.1805112102060.49711@puchar.net> References: <CAA3ZYrCtsLSgW6eA0akOD6Upyx1cTt5qbgaY9iSeguQuQmo%2Bjg@mail.gmail.com> <alpine.BSF.2.20.1805112102060.49711@puchar.net>
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On Fri, May 11, 2018 at 1:03 PM, Wojciech Puchar <wojtek@puchar.net> wrote: > There are also PCIe cards which provide multiple slots, typically >> connected with a usb cable. These tend to convert 1 PCIe_x1 slot >> into multiple PCIe_x1 slots. I get the impression that these do >> not require bifurcation support. They seem to be aimed at "miners" >> > > these cards consist of PCIe switch which is supported out of the box. > former ones require BIOS to reconfigure CPUs PCI lanes so instead of eg > one 16x lane there will be 4 4x lanes. > Usually they require a driver for management functions, like if you want to turn off one of the ports (because you know there's a bad card in it, for example). But for normal probe / attach they are usually drop in. Some mobos can require some BIOS adjustment, though to properly setup lane bifurcation and such... The ones that are true switches, and not just signal retimers, usually don't: they take the full 8 or 16 lanes and multiplex it amongst the 16-32 downstream lanes they typically provide. We're looking at one that does x4 lanes and expands to 4 cards at x4 lanes. The devices on the other side fill just over a lane each, but we'd run out of lanes if we did x2 fan-out. This the cards we're looking at, we can get x4 combined rates form the 4 cards that on their own are kinda crappy. Warner
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