linux-kernel - [RFC PATCH 6/6] cxl/region, dax/hmem: Guard CXL DAX region creation and tighten HMEM deps

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Message-ID: <20250822034202.26896-7-Smita.KoralahalliChannabasappa@amd.com>
Date: Fri, 22 Aug 2025 03:42:02 +0000
From: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>
To: <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<nvdimm@...ts.linux.dev>, <linux-fsdevel@...r.kernel.org>,
	<linux-pm@...r.kernel.org>
CC: Davidlohr Bueso <dave@...olabs.net>, Jonathan Cameron
	<jonathan.cameron@...wei.com>, Dave Jiang <dave.jiang@...el.com>, "Alison
 Schofield" <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>, Dan Williams
	<dan.j.williams@...el.com>, Matthew Wilcox <willy@...radead.org>, Jan Kara
	<jack@...e.cz>, "Rafael J . Wysocki" <rafael@...nel.org>, Len Brown
	<len.brown@...el.com>, Pavel Machek <pavel@...nel.org>, Li Ming
	<ming.li@...omail.com>, Jeff Johnson <jeff.johnson@....qualcomm.com>, "Ying
 Huang" <huang.ying.caritas@...il.com>, Yao Xingtao <yaoxt.fnst@...itsu.com>,
	Peter Zijlstra <peterz@...radead.org>, Greg KH <gregkh@...uxfoundation.org>,
	Nathan Fontenot <nathan.fontenot@....com>, Smita Koralahalli
	<Smita.KoralahalliChannabasappa@....com>, Terry Bowman
	<terry.bowman@....com>, Robert Richter <rrichter@....com>, Benjamin Cheatham
	<benjamin.cheatham@....com>, PradeepVineshReddy Kodamati
	<PradeepVineshReddy.Kodamati@....com>, Zhijian Li <lizhijian@...itsu.com>
Subject: [RFC PATCH 6/6] cxl/region, dax/hmem: Guard CXL DAX region creation and tighten HMEM deps

Prevent cxl_region_probe() from unconditionally calling into
devm_cxl_add_dax_region() when the DEV_DAX_CXL driver is not enabled.
Wrap the call with IS_ENABLED(CONFIG_DEV_DAX_CXL) so region probe skips
DAX setup cleanly if no consumer is present.

In parallel, update DEV_DAX_HMEM’s Kconfig to depend on
!CXL_BUS || (CXL_ACPI && CXL_PCI) || m. This ensures:

Built-in (y) HMEM is allowed when CXL is disabled, or when the full
CXL discovery stack is built-in. Module (m) HMEM remains always possible.

Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>
---
I did not want to override Dan’s original approach, so I am posting this
as an RFC.

This patch addresses a corner case when applied on top of Patches 1–5.

When DEV_DAX_HMEM=y and CXL=m, the DEV_DAX_CXL option ends up disabled.
In that configuration, with Patches 1–5 applied, ownership of the Soft
Reserved ranges falls back to dax_hmem. As a result, /proc/iomem looks
like this:

850000000-284fffffff : CXL Window 0
  850000000-284fffffff : region3
    850000000-284fffffff : Soft Reserved
      850000000-284fffffff : dax0.0
        850000000-284fffffff : System RAM (kmem)
2850000000-484fffffff : CXL Window 1
  2850000000-484fffffff : region4
    2850000000-484fffffff : Soft Reserved
      2850000000-484fffffff : dax1.0
        2850000000-484fffffff : System RAM (kmem)
4850000000-684fffffff : CXL Window 2
  4850000000-684fffffff : region5
    4850000000-684fffffff : Soft Reserved
      4850000000-684fffffff : dax2.0
        4850000000-684fffffff : System RAM (kmem)

In this case the dax devices are created by dax_hmem, not by dax_cxl.
Consequently, a "cxl disable-region <regionx>" operation does not
unregister these devices. In addition, the dmesg output can be misleading
to users, since it looks like the CXL region driver created the devdax
devices:

  devm_cxl_add_region: cxl_acpi ACPI0017:00: decoder0.2: created region5
  ..
  ..

This patch addresses those situations. I am not entirely sure how clean
the approach of using “|| m” is, so I am sending it as RFC for feedback.
---
 drivers/cxl/core/region.c | 4 +++-
 drivers/dax/Kconfig       | 1 +
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 71cc42d05248..6a2c21e55dbc 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -3617,7 +3617,9 @@ static int cxl_region_probe(struct device *dev)
 					p->res->start, p->res->end, cxlr,
 					is_system_ram) > 0)
 			return 0;
-		return devm_cxl_add_dax_region(cxlr);
+		if (IS_ENABLED(CONFIG_DEV_DAX_CXL))
+			return devm_cxl_add_dax_region(cxlr);
+		return 0;
 	default:
 		dev_dbg(&cxlr->dev, "unsupported region mode: %d\n",
 			cxlr->mode);
diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
index 3683bb3f2311..fd12cca91c78 100644
--- a/drivers/dax/Kconfig
+++ b/drivers/dax/Kconfig
@@ -30,6 +30,7 @@ config DEV_DAX_PMEM
 config DEV_DAX_HMEM
 	tristate "HMEM DAX: direct access to 'specific purpose' memory"
 	depends on EFI_SOFT_RESERVE
+	depends on !CXL_BUS || (CXL_ACPI && CXL_PCI) || m
 	select NUMA_KEEP_MEMINFO if NUMA_MEMBLKS
 	default DEV_DAX
 	help
-- 
2.17.1