Re: [RFC PATCH] resource: Fix CXL node not populated issue

Bharata B Rao
2024-12-11T16:47:26Z

On 11-Dec-24 2:08 PM, Dan Williams wrote:
> Ah, thanks for that!
>
> So, it turns out Ying's patch is working as advertised. It is traversing
> past the top-level entry of the the iomem_resource topology to discover that a
> Soft Reserved range is described by CXL. Then the expectation is that
> the CXL subsystem parses the topology and registers a dax device.
>
> I missed that detail in my repro because I was not testing the HMEM
> handoff.
>
> Now, the dmesg shows that the CXL subsystem gives up early on the
> CXL devices as they do not appear to be meeting the expecations of a "CXL
> Memory Device" as described by the "PCI Header - Class Code Register
> (Offset 09h)" of the CXL spec.
>
> Specifically these messages:
>
> [ 4.449072] cxl_pci 0000:9f:00.0: registers not found: status mbox memdev
>
> I think this situation is increasingly showing that the pace of
> non-standard quirks being deployed is higher than CXL subsystem is able
> to keep pace.
>
> The immediate workaround to this problem that Linux discovered is to
> disable the cxl_acpi driver. Can you confirm that preventing the
> cxl_acpi driver from loading restores that missing node?

Yes, disabling CONFIG_CXL_ACPI gets the CXL node up with the memory.
Thanks for this workaround.

>
> Longer term the urgency of Nathan's patch [1] needs to be escalated.
>
> [1]: http://lore.kernel.org/20241202155542.22111-1-nathan.fontenot@amd.com

I didn't find that patch helping though with CONFIG_CXL_ACPI kept enabled.

Regards,
Bharata.