linux-kernel - Re: [PATCH v4 12/14] cxl: Simplify cxl_rd

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Message-ID: <aQqF-xCnL2Ugh3hp@aschofie-mobl2.lan>
Date: Tue, 4 Nov 2025 15:02:19 -0800
From: Alison Schofield <alison.schofield@...el.com>
To: Robert Richter <rrichter@....com>
CC: Vishal Verma <vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>,
	Dan Williams <dan.j.williams@...el.com>, Jonathan Cameron
	<Jonathan.Cameron@...wei.com>, Dave Jiang <dave.jiang@...el.com>, "Davidlohr
 Bueso" <dave@...olabs.net>, <linux-cxl@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Gregory Price <gourry@...rry.net>, "Fabio M.
 De Francesco" <fabio.m.de.francesco@...ux.intel.com>, Terry Bowman
	<terry.bowman@....com>, Joshua Hahn <joshua.hahnjy@...il.com>
Subject: Re: [PATCH v4 12/14] cxl: Simplify cxl_rd_ops allocation and handling

On Mon, Nov 03, 2025 at 07:47:53PM +0100, Robert Richter wrote:
> A root decoder's callback handlers are collected in struct cxl_rd_ops.
> The structure is dynamically allocated, though it contains only a few
> pointers in it. This also requires to check two pointes to check for
> the existance of a callback.
> 
> Simplify the allocation, release and handler check by embedding the
> ops statical in struct cxl_root_decoder.
> 
> Implementation is equivalent to how struct cxl_root_ops handles the
> callbacks.

The allocation was intentionally dynamic because the root decoder ops
only existed for CFMWS's defined w XOR Arithmetic.

>From the commit msg:
>> To avoid maintaining a static ops instance populated with mostly NULL
>> pointers, allocate the ops structure dynamically only when a platform
>> requires overrides (e.g. XOR interleave decoding).
>> The setup can be extended as additional callbacks are added.
See: 524b2b76f365 ("cxl: Move hpa_to_spa callback to a new root decoder ops structure")

Has the usage changed?

-- Alison


> 
> Signed-off-by: Robert Richter <rrichter@....com>
> ---
>  drivers/cxl/acpi.c        |  8 ++------
>  drivers/cxl/core/region.c | 20 +++++---------------
>  drivers/cxl/cxl.h         |  2 +-
>  3 files changed, 8 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> index f9bbc77f3ec2..778ee29430ea 100644
> --- a/drivers/cxl/acpi.c
> +++ b/drivers/cxl/acpi.c
> @@ -471,12 +471,8 @@ static int __cxl_parse_cfmws(struct acpi_cedt_cfmws *cfmws,
>  	cxlrd->qos_class = cfmws->qtg_id;
>  
>  	if (cfmws->interleave_arithmetic == ACPI_CEDT_CFMWS_ARITHMETIC_XOR) {
> -		cxlrd->ops = kzalloc(sizeof(*cxlrd->ops), GFP_KERNEL);
> -		if (!cxlrd->ops)
> -			return -ENOMEM;
> -
> -		cxlrd->ops->hpa_to_spa = cxl_apply_xor_maps;
> -		cxlrd->ops->spa_to_hpa = cxl_apply_xor_maps;
> +		cxlrd->ops.hpa_to_spa = cxl_apply_xor_maps;
> +		cxlrd->ops.spa_to_hpa = cxl_apply_xor_maps;
>  	}
>  
>  	rc = cxl_decoder_add(cxld);
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 379a67cc8e31..dec003084521 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -2932,16 +2932,6 @@ static bool cxl_is_hpa_in_chunk(u64 hpa, struct cxl_region *cxlr, int pos)
>  	return false;
>  }
>  
> -static bool has_hpa_to_spa(struct cxl_root_decoder *cxlrd)
> -{
> -	return cxlrd->ops && cxlrd->ops->hpa_to_spa;
> -}
> -
> -static bool has_spa_to_hpa(struct cxl_root_decoder *cxlrd)
> -{
> -	return cxlrd->ops && cxlrd->ops->spa_to_hpa;
> -}
> -
>  u64 cxl_dpa_to_hpa(struct cxl_region *cxlr, const struct cxl_memdev *cxlmd,
>  		   u64 dpa)
>  {
> @@ -2996,8 +2986,8 @@ u64 cxl_dpa_to_hpa(struct cxl_region *cxlr, const struct cxl_memdev *cxlmd,
>  	hpa = hpa_offset + p->res->start + p->cache_size;
>  
>  	/* Root decoder translation overrides typical modulo decode */
> -	if (has_hpa_to_spa(cxlrd))
> -		hpa = cxlrd->ops->hpa_to_spa(cxlrd, hpa);
> +	if (cxlrd->ops.hpa_to_spa)
> +		hpa = cxlrd->ops.hpa_to_spa(cxlrd, hpa);
>  
>  	if (!cxl_resource_contains_addr(p->res, hpa)) {
>  		dev_dbg(&cxlr->dev,
> @@ -3006,7 +2996,7 @@ u64 cxl_dpa_to_hpa(struct cxl_region *cxlr, const struct cxl_memdev *cxlmd,
>  	}
>  
>  	/* Simple chunk check, by pos & gran, only applies to modulo decodes */
> -	if (!has_hpa_to_spa(cxlrd) && (!cxl_is_hpa_in_chunk(hpa, cxlr, pos)))
> +	if (!cxlrd->ops.hpa_to_spa && !cxl_is_hpa_in_chunk(hpa, cxlr, pos))
>  		return ULLONG_MAX;
>  
>  	return hpa;
> @@ -3041,8 +3031,8 @@ static int region_offset_to_dpa_result(struct cxl_region *cxlr, u64 offset,
>  	 * If the root decoder has SPA to CXL HPA callback, use it. Otherwise
>  	 * CXL HPA is assumed to equal SPA.
>  	 */
> -	if (has_spa_to_hpa(cxlrd)) {
> -		hpa = cxlrd->ops->spa_to_hpa(cxlrd, p->res->start + offset);
> +	if (cxlrd->ops.spa_to_hpa) {
> +		hpa = cxlrd->ops.spa_to_hpa(cxlrd, p->res->start + offset);
>  		hpa_offset = hpa - p->res->start;
>  	} else {
>  		hpa_offset = offset;
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index 0af46d1b0abc..75fd45ddca38 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -451,7 +451,7 @@ struct cxl_root_decoder {
>  	void *platform_data;
>  	struct mutex range_lock;
>  	int qos_class;
> -	struct cxl_rd_ops *ops;
> +	struct cxl_rd_ops ops;
>  	struct cxl_switch_decoder cxlsd;
>  };
>  
> -- 
> 2.47.3
>