Re: [PATCH v4 7/7] cxl/dax: Defer DAX consumption of SOFT RESERVED resources until after CXL region creation

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Tue, 3 Jun 2025 22:19:49 +0000
Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com> wrote:

> From: Nathan Fontenot <nathan.fontenot@amd.com>
> 
> The DAX HMEM driver currently consumes all SOFT RESERVED iomem resources
> during initialization. This interferes with the CXL driver’s ability to
> create regions and trim overlapping SOFT RESERVED ranges before DAX uses
> them.
> 
> To resolve this, defer the DAX driver's resource consumption if the
> cxl_acpi driver is enabled. The DAX HMEM initialization skips walking the
> iomem resource tree in this case. After CXL region creation completes,
> any remaining SOFT RESERVED resources are explicitly registered with the
> DAX driver by the CXL driver.
> 
> This sequencing ensures proper handling of overlaps and fixes hotplug
> failures.
> 
> Co-developed-by: Nathan Fontenot <Nathan.Fontenot@amd.com>
> Signed-off-by: Nathan Fontenot <Nathan.Fontenot@amd.com>
> Co-developed-by: Terry Bowman <terry.bowman@amd.com>
> Signed-off-by: Terry Bowman <terry.bowman@amd.com>
> Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
> ---
>  drivers/cxl/core/region.c | 10 +++++++++
>  drivers/dax/hmem/device.c | 43 ++++++++++++++++++++-------------------
>  drivers/dax/hmem/hmem.c   |  3 ++-
>  include/linux/dax.h       |  6 ++++++
>  4 files changed, 40 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 3a5ca44d65f3..c6c0c7ba3b20 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -10,6 +10,7 @@
>  #include <linux/sort.h>
>  #include <linux/idr.h>
>  #include <linux/memory-tiers.h>
> +#include <linux/dax.h>
>  #include <cxlmem.h>
>  #include <cxl.h>
>  #include "core.h"
> @@ -3553,6 +3554,11 @@ static struct resource *normalize_resource(struct resource *res)
>  	return NULL;
>  }
>  
> +static int cxl_softreserv_mem_register(struct resource *res, void *unused)
> +{
> +	return hmem_register_device(phys_to_target_node(res->start), res);
> +}
> +
>  static int __cxl_region_softreserv_update(struct resource *soft,
>  					  void *_cxlr)
>  {
> @@ -3590,6 +3596,10 @@ int cxl_region_softreserv_update(void)
>  				    __cxl_region_softreserv_update);
>  	}
>  
> +	/* Now register any remaining SOFT RESERVES with DAX */
> +	walk_iomem_res_desc(IORES_DESC_SOFT_RESERVED, IORESOURCE_MEM,
> +			    0, -1, NULL, cxl_softreserv_mem_register);
> +
>  	return 0;
>  }
>  EXPORT_SYMBOL_NS_GPL(cxl_region_softreserv_update, "CXL");
> diff --git a/drivers/dax/hmem/device.c b/drivers/dax/hmem/device.c
> index 59ad44761191..cc1ed7bbdb1a 100644
> --- a/drivers/dax/hmem/device.c
> +++ b/drivers/dax/hmem/device.c
> @@ -8,7 +8,6 @@
>  static bool nohmem;
>  module_param_named(disable, nohmem, bool, 0444);
>  
> -static bool platform_initialized;
>  static DEFINE_MUTEX(hmem_resource_lock);
>  static struct resource hmem_active = {
>  	.name = "HMEM devices",
> @@ -35,9 +34,7 @@ EXPORT_SYMBOL_GPL(walk_hmem_resources);
>  
>  static void __hmem_register_resource(int target_nid, struct resource *res)
>  {
> -	struct platform_device *pdev;
>  	struct resource *new;
> -	int rc;
>  
>  	new = __request_region(&hmem_active, res->start, resource_size(res), "",
>  			       0);
> @@ -47,21 +44,6 @@ static void __hmem_register_resource(int target_nid, struct resource *res)
>  	}
>  
>  	new->desc = target_nid;
> -
> -	if (platform_initialized)
> -		return;
> -
> -	pdev = platform_device_alloc("hmem_platform", 0);
> -	if (!pdev) {
> -		pr_err_once("failed to register device-dax hmem_platform device\n");
> -		return;
> -	}
> -
> -	rc = platform_device_add(pdev);
> -	if (rc)
> -		platform_device_put(pdev);
> -	else
> -		platform_initialized = true;
>  }
>  
>  void hmem_register_resource(int target_nid, struct resource *res)
> @@ -83,9 +65,28 @@ static __init int hmem_register_one(struct resource *res, void *data)
>  
>  static __init int hmem_init(void)
>  {
> -	walk_iomem_res_desc(IORES_DESC_SOFT_RESERVED,
> -			IORESOURCE_MEM, 0, -1, NULL, hmem_register_one);
> -	return 0;
> +	struct platform_device *pdev;
> +	int rc;
> +
> +	if (!IS_ENABLED(CONFIG_CXL_ACPI)) {
> +		walk_iomem_res_desc(IORES_DESC_SOFT_RESERVED,
> +				    IORESOURCE_MEM, 0, -1, NULL,
> +				    hmem_register_one);
> +	}
> +
> +	pdev = platform_device_alloc("hmem_platform", 0);
> +	if (!pdev) {
> +		pr_err("failed to register device-dax hmem_platform device\n");
> +		return -1;
> +	}
> +
> +	rc = platform_device_add(pdev);

platform_device_register_simple("hmem_platform", -1, NULL, 0); or something like
that?  There are quite a few variants of platform_device_register to cover
simple cases.


> +	if (rc) {
> +		pr_err("failed to add device-dax hmem_platform device\n");
> +		platform_device_put(pdev);
> +	}
> +
> +	return rc;
>  }
>  
>  /*