At 2023-10-26 21:35:38, "Jonathan Cameron via groups.io" <jonathan.cameron=huawei.com@groups.io> wrote: >On Thu, 26 Oct 2023 11:49:28 +0200 >"Laszlo Ersek" <lersek@redhat.com> wrote: > >> On 10/26/23 10:33, Gerd Hoffmann wrote: >> > On Thu, Oct 26, 2023 at 10:36:35AM +0800, Yoshinoya wrote: >> > >> >> CXL Host Bridge / Root Port / Switch / Device enumeration / HDM Config, maybe could be integrated into pci drivers stack. >> > >> > Point being? Can or should the firmware do anything useful with >> > the CXL hardware? If so, what exactly and why? >> > >> > Current state of affairs is that the PCI stack does the usual PCI >> > initialization (enumerate, assign resources to PCI bars) and leaves >> > everything else to the OS. >> >> (I don't know what "HDM Config" stands for.) >> >> The only utility for driving CXL devices from the firmware could be, AFAICT: >> >> - booting off of such a device (or at least "supporting OS boot" in some >> manner) >> >> - using such a device for UEFI console purposes > >There are different models for how to use CXL devices and what's possible depends on the >version of CXL. CXL 1.1 wasn't great for standards defined discovery, so >EDK2 platform logic basically has to do everything. > >The one mostly expected for early CXL servers, for backwards compatibility, makes >setting up the CXL memory decoders (Host managed Device Memory - HDM) in all the >components in the path to memory + locking them down an EDK2 problem. They are then >presented in the memory map and in SRAT, HMAT etc the same as normal DDR memory. >Idea being that an old OS will be fine with that and doesn't have to be CXL aware >at all. Note this also involves walking the CDAT tables via DOE mailboxes in PCI >config space to get the magic numbers needed to compute HMAT. > >The other model is to do very little in EDK2 and make entirely a problem for the OS. >The logic is necessary anyway if you want to support hotplug etc, so use it for the >cold plug paths 2. That's all we've currently supported on QEMU. > >There was a presentation at Linux Plumbers last year on some out of tree support >from ARM for doing the setup on a CXL 2.0 platform (I think) in EDK2 >https://lpc.events/event/16/contributions/1254/ >But I guess it never went upstream. >https://github.com/SayantaP-arm/edk2-platforms/tree/cxl-type-3 > >+CC Sayanta > >Jonathan >> >> Laszlo >> >> >> >> >> >> > > > > >
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