linux-kernel - Re: [PATCH v4 07/14] cxl/region: Use region data to get the root decoder

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Message-ID: <288d310a-af1f-47ce-ab03-ac57f3901793@intel.com>
Date: Mon, 3 Nov 2025 15:30:11 -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>, Joshua Hahn <joshua.hahnjy@...il.com>
Subject: Re: [PATCH v4 07/14] cxl/region: Use region data to get the root
 decoder



On 11/3/25 11:47 AM, Robert Richter wrote:
> To find a region's root decoder, the endpoint's HPA range is used to
> search the matching decoder by its range. With address translation the
> endpoint decoder's range is in a different address space and thus
> cannot be used to determine the root decoder.
> 
> The region parameters are encapsulated within struc cxl_region_context
> and may include the translated Host Physical Address (HPA) range. Use
> this context to identify the root decoder rather than relying on the
> endpoint.
> 
> Modify cxl_find_root_decoder() and add the region context as
> parameter. Rename this function to get_cxl_root_decoder() as a
> counterpart to put_cxl_root_decoder(). Simplify the implementation by
> removing function cxl_port_find_switch_decode(). The function is
> unnecessary because it is not referenced or utilized elsewhere in the
> code.
> 
> Signed-off-by: Robert Richter <rrichter@....com>

Just a nit below. Otherwise
Reviewed-by: Dave Jiang <dave.jiang@...el.com>

> ---
>  drivers/cxl/core/region.c | 50 +++++++++++++++++----------------------
>  1 file changed, 22 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 2cfc29a2b588..2dd9e9be4889 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -3350,47 +3350,39 @@ static int devm_cxl_add_dax_region(struct cxl_region *cxlr)
>  	return rc;
>  }
>  
> -static int match_decoder_by_range(struct device *dev, const void *data)
> +static int match_root_decoder(struct device *dev, const void *data)
>  {
>  	const struct range *r1, *r2 = data;
> -	struct cxl_decoder *cxld;
> +	struct cxl_root_decoder *cxlrd;
>  
> -	if (!is_switch_decoder(dev))
> +	if (!is_root_decoder(dev))
>  		return 0;
>  
> -	cxld = to_cxl_decoder(dev);
> -	r1 = &cxld->hpa_range;
> -	return range_contains(r1, r2);
> -}
> +	cxlrd = to_cxl_root_decoder(dev);
> +	r1 = &cxlrd->cxlsd.cxld.hpa_range;
>  
> -static struct cxl_decoder *
> -cxl_port_find_switch_decoder(struct cxl_port *port, struct range *hpa_range)
> -{
> -	struct device *cxld_dev = device_find_child(&port->dev, hpa_range,
> -						    match_decoder_by_range);
> -
> -	return cxld_dev ? to_cxl_decoder(cxld_dev) : NULL;
> +	return range_contains(r1, r2);
>  }
>  
>  static struct cxl_root_decoder *
> -cxl_find_root_decoder(struct cxl_endpoint_decoder *cxled)
> +get_cxl_root_decoder(struct cxl_endpoint_decoder *cxled,
> +		     struct cxl_region_context *ctx)

Would like a comment that make note the caller should call put_device() when done with the root decoder.

DJ
 >  {
> -	struct cxl_memdev *cxlmd = cxled_to_memdev(cxled);
>  	struct cxl_port *port = cxled_to_port(cxled);
>  	struct cxl_root *cxl_root __free(put_cxl_root) = find_cxl_root(port);
> -	struct cxl_decoder *root, *cxld = &cxled->cxld;
> -	struct range *hpa_range = &cxld->hpa_range;
> +	struct device *cxlrd_dev;
>  
> -	root = cxl_port_find_switch_decoder(&cxl_root->port, hpa_range);
> -	if (!root) {
> -		dev_err(cxlmd->dev.parent,
> +	cxlrd_dev = device_find_child(&cxl_root->port.dev, &ctx->hpa_range,
> +				      match_root_decoder);
> +	if (!cxlrd_dev) {
> +		dev_err(port->uport_dev,
>  			"%s:%s no CXL window for range %#llx:%#llx\n",
> -			dev_name(&cxlmd->dev), dev_name(&cxld->dev),
> -			hpa_range->start, hpa_range->end);
> -		return NULL;
> +			dev_name(&ctx->cxlmd->dev), dev_name(&cxled->cxld.dev),
> +			ctx->hpa_range.start, ctx->hpa_range.end);
> +		return ERR_PTR(-ENXIO);
>  	}
>  
> -	return to_cxl_root_decoder(&root->dev);
> +	return to_cxl_root_decoder(cxlrd_dev);
>  }
>  
>  static int match_region_by_range(struct device *dev, const void *data)
> @@ -3584,9 +3576,11 @@ int cxl_add_to_region(struct cxl_endpoint_decoder *cxled)
>  	};
>  
>  	struct cxl_root_decoder *cxlrd __free(put_cxl_root_decoder) =
> -		cxl_find_root_decoder(cxled);
> -	if (!cxlrd)
> -		return -ENXIO;
> +		get_cxl_root_decoder(cxled, &ctx);
> +
> +	rc = PTR_ERR_OR_ZERO(cxlrd);
> +	if (rc)
> +		return rc;
>  
>  	/*
>  	 * Ensure that if multiple threads race to construct_region()