Re: [edk2-devel] question about cxl device enumeration in pci bus driver

["Jonathan Cameron via groups.io" <jonathan.cameron=huawei.com@groups.io>]

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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