linux-kernel - Re: [PATCH v5 3/7] cxl/acpi: Add background worker to coordinate with cxl

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Message-ID: <68810a42ec985_1196810094@dwillia2-mobl4.notmuch>
Date: Wed, 23 Jul 2025 09:13:54 -0700
From: <dan.j.williams@...el.com>
To: <dan.j.williams@...el.com>, Smita Koralahalli
	<Smita.KoralahalliChannabasappa@....com>, <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: Re: [PATCH v5 3/7] cxl/acpi: Add background worker to coordinate with
 cxl_mem probe completion

dan.j.williams@ wrote:
[..]
> 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< --
> From 48b25461eca050504cf5678afd7837307b2dd14f Mon Sep 17 00:00:00 2001
> From: Dan Williams <dan.j.williams@...el.com>
> Date: Tue, 22 Jul 2025 16:11:08 -0700
> Subject: [RFC PATCH] dax/cxl: Defer Soft Reserved registration

Likely needs this incremental change to prevent DEV_DAX_HMEM from being
built-in when CXL is not. This still leaves the awkward scenario of CXL
enabled, DEV_DAX_CXL disabled, and DEV_DAX_HMEM built-in. I believe that
safely fails in devdax only / fallback mode, but something to
investigate when respinning on top of this.

-- 8< --
diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
index d656e4c0eb84..3683bb3f2311 100644
--- a/drivers/dax/Kconfig
+++ b/drivers/dax/Kconfig
@@ -48,6 +48,8 @@ config DEV_DAX_CXL
 	tristate "CXL DAX: direct access to CXL RAM regions"
 	depends on CXL_BUS && CXL_REGION && DEV_DAX
 	default CXL_REGION && DEV_DAX
+	depends on CXL_ACPI >= DEV_DAX_HMEM
+	depends on CXL_PCI >= DEV_DAX_HMEM
 	help
 	  CXL RAM regions are either mapped by platform-firmware
 	  and published in the initial system-memory map as "System RAM", mapped
diff --git a/drivers/dax/hmem/hmem.c b/drivers/dax/hmem/hmem.c
index 0916478e3817..8bcd104111a8 100644
--- a/drivers/dax/hmem/hmem.c
+++ b/drivers/dax/hmem/hmem.c
@@ -103,7 +103,7 @@ static int hmem_register_device(struct device *host, int target_nid,
 	long id;
 	int rc;
 
-	if (IS_ENABLED(CONFIG_CXL_REGION) &&
+	if (IS_ENABLED(CONFIG_DEV_DAX_CXL) &&
 	    region_intersects(res->start, resource_size(res), IORESOURCE_MEM,
 			      IORES_DESC_CXL) != REGION_DISJOINT) {
 		switch (dax_cxl_mode) {
@@ -209,7 +209,7 @@ static __init int dax_hmem_init(void)
 	 * CXL topology discovery at least once before scanning the
 	 * iomem resource tree for IORES_DESC_CXL resources.
 	 */
-	if (IS_ENABLED(CONFIG_CXL_REGION)) {
+	if (IS_ENABLED(CONFIG_DEV_DAX_CXL)) {
 		request_module("cxl_acpi");
 		request_module("cxl_pci");
 	}