linux-kernel - [PATCH 4/6] cxl: add CONFIG_CXL_REGION_CTRL_AUTO

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Message-ID: <20260112163514.2551809-5-gourry@gourry.net>
Date: Mon, 12 Jan 2026 11:35:12 -0500
From: Gregory Price <gourry@...rry.net>
To: linux-cxl@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	dave@...olabs.net,
	jonathan.cameron@...wei.com,
	dave.jiang@...el.com,
	alison.schofield@...el.com,
	vishal.l.verma@...el.com,
	ira.weiny@...el.com,
	dan.j.williams@...el.com
Subject: [PATCH 4/6] cxl: add CONFIG_CXL_REGION_CTRL_AUTO_* build config options

To give users the option to have the auto-behavior of memory to default
to SYSRAM, provide a switch.  The default is still recommended to be DAX
in case of multiple devices being added to the system, but this provides
simpler systems a path to use the sysram controller for systems already
configured with auto-regions.

Signed-off-by: Gregory Price <gourry@...rry.net>
---
 drivers/cxl/Kconfig                | 32 ++++++++++++++++++++++++++++++
 drivers/cxl/core/memctrl/memctrl.c |  2 ++
 drivers/cxl/cxl.h                  |  2 +-
 3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/drivers/cxl/Kconfig b/drivers/cxl/Kconfig
index 48b7314afdb8..5aed1524f8f1 100644
--- a/drivers/cxl/Kconfig
+++ b/drivers/cxl/Kconfig
@@ -211,6 +211,38 @@ config CXL_REGION
 
 	  If unsure say 'y'
 
+choice
+	prompt "CXL Region Auto Control Mode"
+	depends on CXL_REGION
+	default CXL_REGION_CTRL_AUTO_DAX
+	help
+	  Select the default controller for CXL regions when ctrl mode is
+	  set to 'auto'. This determines how CXL memory regions are exposed
+	  to the system when no explicit control mode is specified.
+
+config CXL_REGION_CTRL_AUTO_DAX
+	bool "DAX"
+	help
+	  When a CXL region's control mode is 'auto', create a DAX region
+	  controller. This allows fine-grained control over the memory region
+	  through the DAX subsystem, and the region can later be converted to
+	  System RAM via daxctl.
+
+	  This is the default and recommended option for most use cases.
+
+config CXL_REGION_CTRL_AUTO_SYSRAM
+	bool "System RAM"
+	help
+	  When a CXL region's control mode is 'auto', hotplug the region
+	  directly as System RAM. This makes the CXL memory immediately
+	  available to the kernel's memory allocator without requiring
+	  additional userspace configuration.
+
+	  Select this if you want CXL memory to be automatically available
+	  as regular system memory.
+
+endchoice
+
 config CXL_REGION_INVALIDATION_TEST
 	bool "CXL: Region Cache Management Bypass (TEST)"
 	depends on CXL_REGION
diff --git a/drivers/cxl/core/memctrl/memctrl.c b/drivers/cxl/core/memctrl/memctrl.c
index 1b661465bdeb..cb6c37f4c0ee 100644
--- a/drivers/cxl/core/memctrl/memctrl.c
+++ b/drivers/cxl/core/memctrl/memctrl.c
@@ -31,6 +31,8 @@ int cxl_enable_memctrl(struct cxl_region *cxlr)
 					p->res->start, p->res->end, cxlr,
 					is_system_ram) > 0)
 			return 0;
+		if (IS_ENABLED(CONFIG_CXL_REGION_CTRL_AUTO_SYSRAM))
+			return devm_cxl_add_sysram_region(cxlr);
 		return devm_cxl_add_dax_region(cxlr);
 	case CXL_MEMCTRL_DAX:
 		return devm_cxl_add_dax_region(cxlr);
diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
index c69d27a2e97d..1dae6fe4f70c 100644
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -506,7 +506,7 @@ enum cxl_partition_mode {
 /*
  * Memory Controller modes:
  *   None - No controller selected
- *   Auto   - either BIOS-configured as SysRAM, or default to DAX
+ *   Auto   - Auto-select based on BIOS, boot, and build configs.
  *   DAX    - creates a dax_region controller for the cxl_region
  *   SYSRAM - hotplugs the region directly as System RAM
  *   PMEM   - persistent memory controller (nvdimm)
-- 
2.52.0