Re: [PATCH v5 3/7] cxl/acpi: Add background worker to coordinate with cxl_mem probe completion

[dan.j.williams]

Smita Koralahalli wrote:
> Introduce a background worker in cxl_acpi to delay SOFT RESERVE handling
> until the cxl_mem driver has probed at least one device. This coordination
> ensures that DAX registration or fallback handling for soft-reserved
> regions is not triggered prematurely.
> 
> The worker waits on cxl_wait_queue, which is signaled via
> cxl_mem_active_inc() during cxl_mem_probe(). Once at least one memory
> device probe is confirmed, the worker invokes wait_for_device_probe()
> to allow the rest of the CXL device hierarchy to complete initialization.
> 
> Additionally, it also handles initialization order issues where
> cxl_acpi_probe() may complete before other drivers such as cxl_port or
> cxl_mem have loaded, especially when cxl_acpi and cxl_port are built-in
> and cxl_mem is a loadable module. In such cases, using only
> wait_for_device_probe() is insufficient, as it may return before all
> relevant probes are registered.

Right, but that problem is not solved by this which still leaves the
decision on when to give up on this mechanism, and this mechanism does
not tell you when follow-on probe work is complete.

> While region creation happens in cxl_port_probe(), waiting on
> cxl_mem_active() would be sufficient as cxl_mem_probe() can only succeed
> after the port hierarchy is in place. Furthermore, since cxl_mem depends
> on cxl_pci, this also guarantees that cxl_pci has loaded by the time the
> wait completes.
> 
> As cxl_mem_active() infrastructure already exists for tracking probe
> activity, cxl_acpi can use it without introducing new coordination
> mechanisms.

In appreciate the instinct to not add anything new, but the module
loading problem is solvable.

If the goal is: "I want to give device-dax a point at which it can make
a go / no-go decision about whether the CXL subsystem has properly
assembled all CXL regions implied by Soft Reserved instersecting with
CXL Windows." Then that is something like the below, only lightly tested
and likely regresses the non-CXL case.

-- 8< --