Re: [PATCH v4 6/9] cxl/region: Add register_dax flag to defer DAX setup

["Koralahalli Channabasappa, Smita" <skoralah@amd.com>]

On 11/19/2025 7:19 PM, Smita Koralahalli wrote:
> Stop creating cxl_dax during cxl_region_probe(). Early DAX registration
> can online memory before ownership of Soft Reserved ranges is finalized.
> This makes it difficult to tear down regions later when HMEM determines
> that a region should not claim that range.
> 
> Introduce a register_dax flag in struct cxl_region_params and gate DAX
> registration on this flag. Leave probe time registration disabled for
> regions discovered during early CXL enumeration; set the flag only for
> regions created dynamically at runtime to preserve existing behaviour.
> 
> This patch prepares the region code for later changes where cxl_dax
> setup occurs from the HMEM path only after ownership arbitration
> completes.
> 
> Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
> ---
>   drivers/cxl/core/region.c | 21 ++++++++++++++++-----
>   drivers/cxl/cxl.h         |  1 +
>   2 files changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 94dbbd6b5513..c17cd8706b9d 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -2540,9 +2540,11 @@ static int cxl_region_calculate_adistance(struct notifier_block *nb,
>   static struct cxl_region *devm_cxl_add_region(struct cxl_root_decoder *cxlrd,
>   					      int id,
>   					      enum cxl_partition_mode mode,
> -					      enum cxl_decoder_type type)
> +					      enum cxl_decoder_type type,
> +					      bool register_dax)
>   {
>   	struct cxl_port *port = to_cxl_port(cxlrd->cxlsd.cxld.dev.parent);
> +	struct cxl_region_params *p;
>   	struct cxl_region *cxlr;
>   	struct device *dev;
>   	int rc;
> @@ -2553,6 +2555,9 @@ static struct cxl_region *devm_cxl_add_region(struct cxl_root_decoder *cxlrd,
>   	cxlr->mode = mode;
>   	cxlr->type = type;
>   
> +	p = &cxlr->params;
> +	p->register_dax = register_dax;
> +
>   	dev = &cxlr->dev;
>   	rc = dev_set_name(dev, "region%d", id);
>   	if (rc)
> @@ -2593,7 +2598,8 @@ static ssize_t create_ram_region_show(struct device *dev,
>   }
>   
>   static struct cxl_region *__create_region(struct cxl_root_decoder *cxlrd,
> -					  enum cxl_partition_mode mode, int id)
> +					  enum cxl_partition_mode mode, int id,
> +					  bool register_dax)
>   {
>   	int rc;
>   
> @@ -2615,7 +2621,8 @@ static struct cxl_region *__create_region(struct cxl_root_decoder *cxlrd,
>   		return ERR_PTR(-EBUSY);
>   	}
>   
> -	return devm_cxl_add_region(cxlrd, id, mode, CXL_DECODER_HOSTONLYMEM);
> +	return devm_cxl_add_region(cxlrd, id, mode, CXL_DECODER_HOSTONLYMEM,
> +				   register_dax);
>   }
>   
>   static ssize_t create_region_store(struct device *dev, const char *buf,
> @@ -2629,7 +2636,7 @@ static ssize_t create_region_store(struct device *dev, const char *buf,
>   	if (rc != 1)
>   		return -EINVAL;
>   
> -	cxlr = __create_region(cxlrd, mode, id);
> +	cxlr = __create_region(cxlrd, mode, id, true);
>   	if (IS_ERR(cxlr))
>   		return PTR_ERR(cxlr);
>   
> @@ -3523,7 +3530,7 @@ static struct cxl_region *construct_region(struct cxl_root_decoder *cxlrd,
>   
>   	do {
>   		cxlr = __create_region(cxlrd, cxlds->part[part].mode,
> -				       atomic_read(&cxlrd->region_id));
> +				       atomic_read(&cxlrd->region_id), false);
>   	} while (IS_ERR(cxlr) && PTR_ERR(cxlr) == -EBUSY);
>   
>   	if (IS_ERR(cxlr)) {
> @@ -3930,6 +3937,10 @@ static int cxl_region_probe(struct device *dev)
>   					p->res->start, p->res->end, cxlr,
>   					is_system_ram) > 0)
>   			return 0;
> +
> +		if (!p->register_dax)
> +			return 0;

Sorry, I missed this. It should continue registering DAX if HMEM is 
disabled. I will fix this in v5 and add a comment here

-		if (!p->register_dax)
-			return 0;
+		/*
+		 * Only skip probe time DAX if HMEM will handle it
+		 * later.
+		 */
+		if (IS_ENABLED(CONFIG_DEV_DAX_HMEM) && !p->register_dax)
+			return 0;
> +
>   		return devm_cxl_add_dax_region(cxlr);
>   	default:
>   		dev_dbg(&cxlr->dev, "unsupported region mode: %d\n",
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index af78c9fd37f2..324220596890 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -495,6 +495,7 @@ struct cxl_region_params {
>   	struct cxl_endpoint_decoder *targets[CXL_DECODER_MAX_INTERLEAVE];
>   	int nr_targets;
>   	resource_size_t cache_size;
> +	bool register_dax;
>   };
>   
>   enum cxl_partition_mode {