linux-kernel - RE: [PATCH 15/15] cxl/acpi: Specify module load order dependency for the cxl

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Date:   Fri, 16 Sep 2022 11:12:27 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Robert Richter <rrichter@....com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "Ira Weiny" <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        "Dan Williams" <dan.j.williams@...el.com>
CC:     <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, Robert Richter <rrichter@....com>
Subject: RE: [PATCH 15/15] cxl/acpi: Specify module load order dependency for
 the cxl_acpi module

Robert Richter wrote:
> In RCD mode the CXL mem dev may be detected on the PCI bus before a
> CXL host is brought up. This may cause a CXL mem initialization
> failure as it expects the CXL host already detected.

This is not unique to RCD mode. It is mitigated by the cxl_bus_rescan()
at the completion of cxl_acpi_probe().


> Address this by
> specifying the module dependencies using MODULE_SOFTDEP().
> 
> The following additional dependencies exist:
> 
>  * cxl_mem depends on cxl_acpi: The CXL hosts must be discovered
>    before the CXL device is initialized.
> 
>  * cxl_acpi depends on cxl_port: The acpi driver adds ports to the cxl
>    bus, the port driver should be loaded before. This might also work
>    if modules are loaded in different order, but a) it aligns with the
>    existing cxl_mem/cxl_port softdep and b) it always guarantees a fix
>    module load order.

Why does a fixed module load order matter?

> 
> Signed-off-by: Robert Richter <rrichter@....com>
> ---
>  drivers/cxl/acpi.c | 1 +
>  drivers/cxl/mem.c  | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> index 56b2d222afcc..63a1cb295c07 100644
> --- a/drivers/cxl/acpi.c
> +++ b/drivers/cxl/acpi.c
> @@ -834,3 +834,4 @@ module_exit(cxl_host_exit);
>  MODULE_LICENSE("GPL v2");
>  MODULE_IMPORT_NS(CXL);
>  MODULE_IMPORT_NS(ACPI);
> +MODULE_SOFTDEP("pre: cxl_port");

The only reason cxl_acpi would need to preload cxl_port is if it wanted
to do something like:

port = devm_cxl_add_port(...);
if (port->dev.driver)
	/* do something with an enabled port */
else
	/* do something else if the port failed to enable */

> diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> index 64ccf053d32c..ae13ec7d9894 100644
> --- a/drivers/cxl/mem.c
> +++ b/drivers/cxl/mem.c
> @@ -128,3 +128,4 @@ MODULE_ALIAS_CXL(CXL_DEVICE_MEMORY_EXPANDER);
>   * endpoint registration.
>   */
>  MODULE_SOFTDEP("pre: cxl_port");
> +MODULE_SOFTDEP("pre: cxl_acpi");

There is no strict requirement that CXL topologies be limited to ACPI
platforms. Per above, the cxl_mem driver will attach when the CXL root
device finally appears, and async out-of-order arrival is ok.