FAILED: Patch "cxl/port: Fix CXL port initialization order when the subsystem is built-in" failed to apply to v6.6-stable tree

The patch below does not apply to the v6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 6575b268157f37929948a8d1f3bafb3d7c055bc1 Mon Sep 17 00:00:00 2001
From: Dan Williams <[email protected]>
Date: Fri, 25 Oct 2024 12:32:55 -0700
Subject: [PATCH] cxl/port: Fix CXL port initialization order when the
subsystem is built-in

When the CXL subsystem is built-in the module init order is determined by Makefile order. That order violates expectations. The expectation is that cxl_acpi and cxl_mem can race to attach. If cxl_acpi wins the race, cxl_mem will find the enabled CXL root ports it needs. If cxl_acpi loses the race it will retrigger cxl_mem to attach via cxl_bus_rescan(). That flow only works if cxl_acpi can assume ports are enabled immediately upon cxl_acpi_probe() return. That in turn can only happen in the CONFIG_CXL_ACPI=y case if the cxl_port driver is registered before cxl_acpi_probe() runs.

Fix up the order to prevent initialization failures. Ensure that cxl_port is built-in when cxl_acpi is also built-in, arrange for Makefile order to resolve the subsys_initcall() order of cxl_port and cxl_acpi, and arrange for Makefile order to resolve the device_initcall() (module_init()) order of the remaining objects.

As for what contributed to this not being found earlier, the CXL regression environment, cxl_test, builds all CXL functionality as a module to allow to symbol mocking and other dynamic reload tests. As a result there is no regression coverage for the built-in case.

---
 drivers/cxl/Kconfig  |  1 +
 drivers/cxl/Makefile | 20 ++++++++++++++------
 drivers/cxl/port.c   | 17 ++++++++++++++++-
 3 files changed, 31 insertions(+), 7 deletions(-)

index 29c192f20082c..876469e23f7a7 100644
@@ -60,6 +60,7 @@ config CXL_ACPI
 	default CXL_BUS
 	select ACPI_TABLE_LIB
 	select ACPI_HMAT
+	select CXL_PORT
 	help
 	  Enable support for host managed device memory (HDM) resources
 	  published by a platform's ACPI CXL memory layout description.  See

index db321f48ba52e..2caa90fa4bf25 100644
@@ -1,13 +1,21 @@
 # SPDX-License-Identifier: GPL-2.0
+
+# Order is important here for the built-in case:
+# - 'core' first for fundamental init
+# - 'port' before platform root drivers like 'acpi' so that CXL-root ports
+#   are immediately enabled
+# - 'mem' and 'pmem' before endpoint drivers so that memdevs are
+#   immediately enabled
+# - 'pci' last, also mirrors the hardware enumeration hierarchy
 obj-y += core/
-obj-$(CONFIG_CXL_PCI) += cxl_pci.o
-obj-$(CONFIG_CXL_MEM) += cxl_mem.o
+obj-$(CONFIG_CXL_PORT) += cxl_port.o
 obj-$(CONFIG_CXL_ACPI) += cxl_acpi.o
 obj-$(CONFIG_CXL_PMEM) += cxl_pmem.o
-obj-$(CONFIG_CXL_PORT) += cxl_port.o
+obj-$(CONFIG_CXL_MEM) += cxl_mem.o
+obj-$(CONFIG_CXL_PCI) += cxl_pci.o
 
-cxl_mem-y := mem.o
-cxl_pci-y := pci.o
+cxl_port-y := port.o
 cxl_acpi-y := acpi.o
 cxl_pmem-y := pmem.o security.o
-cxl_port-y := port.o
+cxl_mem-y := mem.o
+cxl_pci-y := pci.o

index 861dde65768fe..9dc394295e1fc 100644
@@ -208,7 +208,22 @@ static struct cxl_driver cxl_port_driver = {
 	},
 };
 
-module_cxl_driver(cxl_port_driver);
+static int __init cxl_port_init(void)
+{
+	return cxl_driver_register(&cxl_port_driver);
+}
+/*
+ * Be ready to immediately enable ports emitted by the platform CXL root
+ * (e.g. cxl_acpi) when CONFIG_CXL_PORT=y.
+ */
+subsys_initcall(cxl_port_init);
+
+static void __exit cxl_port_exit(void)
+{
+	cxl_driver_unregister(&cxl_port_driver);
+}
+module_exit(cxl_port_exit);
+
 MODULE_DESCRIPTION("CXL: Port enumeration and services");
 MODULE_LICENSE("GPL v2");
 MODULE_IMPORT_NS(CXL);

2.43.0