linux-kernel - Re: [PATCH v3 13/18] cxl/region: Add function to find a port's switch decoder by range

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Message-ID: <e5cdb4b2-9be4-4af5-9579-01451d221c74@intel.com>
Date: Thu, 20 Feb 2025 10:23:51 -0700
From: Dave Jiang <dave.jiang@...el.com>
To: Robert Richter <rrichter@....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>,
 Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 Davidlohr Bueso <dave@...olabs.net>
Cc: 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>
Subject: Re: [PATCH v3 13/18] cxl/region: Add function to find a port's switch
 decoder by range



On 2/11/25 2:53 AM, Robert Richter wrote:
> Factor out code to find the switch decoder of a port for a specific
> address range. Reuse the code to search a root decoder, create the
> function cxl_port_find_switch_decoder() and rework
> match_root_decoder_by_range() to be usable for switch decoders too.
> 
> Signed-off-by: Robert Richter <rrichter@....com>
> Reviewed-by: Gregory Price <gourry@...rry.net>
> Tested-by: Gregory Price <gourry@...rry.net>
Reviewed-by: Dave Jiang <dave.jiang@...el.com>

> ---
>  drivers/cxl/core/region.c | 48 +++++++++++++++++++++++----------------
>  1 file changed, 28 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index cfcd235f311e..72e991e7d9ab 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -3189,28 +3189,44 @@ static int devm_cxl_add_dax_region(struct cxl_region *cxlr)
>  	return rc;
>  }
>  
> -static int match_root_decoder_by_range(struct device *dev,
> -				       const void *data)
> +static int match_decoder_by_range(struct device *dev, const void *data)
>  {
>  	const struct range *r1, *r2 = data;
> -	struct cxl_root_decoder *cxlrd;
> +	struct cxl_decoder *cxld;
>  
> -	if (!is_root_decoder(dev))
> +	if (!is_switch_decoder(dev))
>  		return 0;
>  
> -	cxlrd = to_cxl_root_decoder(dev);
> -	r1 = &cxlrd->cxlsd.cxld.hpa_range;
> +	cxld = to_cxl_decoder(dev);
> +	r1 = &cxld->hpa_range;
>  	return range_contains(r1, r2);
>  }
>  
> +static struct cxl_decoder *
> +cxl_port_find_switch_decoder(struct cxl_port *port, struct range *hpa)
> +{
> +	/*
> +	 * device_find_child() increments the reference count of the
> +	 * the switch decoder's parent port to protect the reference
> +	 * to its child. The port is already a parent of the endpoint
> +	 * decoder's port, at least indirectly in the port hierarchy.
> +	 * Thus, the endpoint already holds a reference for the parent
> +	 * port of the switch decoder. Free the unnecessary reference
> +	 * here.
> +	 */
> +	struct device *cxld_dev __free(put_device) =
> +		device_find_child(&port->dev, hpa, match_decoder_by_range);
> +
> +	return cxld_dev ? to_cxl_decoder(cxld_dev) : NULL;
> +}
> +
>  static struct cxl_root_decoder *
>  cxl_find_root_decoder(struct cxl_endpoint_decoder *cxled)
>  {
>  	struct cxl_memdev *cxlmd = cxled_to_memdev(cxled);
>  	struct cxl_port *iter = cxled_to_port(cxled);
> -	struct cxl_decoder *cxld = &cxled->cxld;
> +	struct cxl_decoder *root, *cxld = &cxled->cxld;
>  	struct range *hpa = &cxld->hpa_range;
> -	struct device *cxlrd_dev;
>  
>  	while (iter && !is_cxl_root(iter))
>  		iter = to_cxl_port(iter->dev.parent);
> @@ -3218,9 +3234,8 @@ cxl_find_root_decoder(struct cxl_endpoint_decoder *cxled)
>  	if (!iter)
>  		return NULL;
>  
> -	cxlrd_dev = device_find_child(&iter->dev, hpa,
> -				      match_root_decoder_by_range);
> -	if (!cxlrd_dev) {
> +	root = cxl_port_find_switch_decoder(iter, hpa);
> +	if (!root) {
>  		dev_err(cxlmd->dev.parent,
>  			"%s:%s no CXL window for range %#llx:%#llx\n",
>  			dev_name(&cxlmd->dev), dev_name(&cxld->dev),
> @@ -3228,16 +3243,9 @@ cxl_find_root_decoder(struct cxl_endpoint_decoder *cxled)
>  		return NULL;
>  	}
>  
> -	/*
> -	 * device_find_child() created a reference to the root
> -	 * decoder. Since the root decoder exists as long as the root
> -	 * port exists and the endpoint already holds a reference to
> -	 * the root port, this additional reference is not needed.
> -	 * Free it here.
> -	 */
> -	put_device(cxlrd_dev);
>  
> -	return to_cxl_root_decoder(cxlrd_dev);
> +
> +	return to_cxl_root_decoder(&root->dev);
>  }
>  
>  static int match_region_by_range(struct device *dev, const void *data)