linux-kernel - Re: [PATCH v1 10/29] cxl/region: Add function to find a port's switch decoder by range

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Message-ID: <2ae858ab-4c79-4101-b1be-026655fc76f5@amd.com>
Date: Fri, 17 Jan 2025 15:31:34 -0600
From: Ben Cheatham <benjamin.cheatham@....com>
To: Robert Richter <rrichter@....com>
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>,
	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>,
	Dave Jiang <dave.jiang@...el.com>, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v1 10/29] cxl/region: Add function to find a port's switch
 decoder by range

On 1/7/25 8:09 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>
> ---
>  drivers/cxl/core/region.c | 43 +++++++++++++++++++++++----------------
>  1 file changed, 25 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 5750ed2796a8..48add814924b 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -3189,19 +3189,35 @@ static int devm_cxl_add_dax_region(struct cxl_region *cxlr)
>  	return rc;
>  }
>  
> -static int match_root_decoder_by_range(struct device *dev, void *data)
> +static int match_decoder_by_range(struct device *dev, void *data)
>  {
>  	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() creates 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.
> +	 */

Is this comment still true? I haven't read the rest of the series yet, but there's
nothing enforcing that this function is called on a root port. If it's meant to
only be used for root ports then it should probably be named that way.

Also, if it is meant to be used for a general switch decoder, can we always free
the reference? If so then all that needs to happen is a comment update, otherwise
you'll need to keep the reference and put a comment somewhere that the function
needs a matching put_device().


> +	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)
>  {
> @@ -3209,7 +3225,6 @@ cxl_find_root_decoder(struct cxl_endpoint_decoder *cxled)
>  	struct cxl_port *iter = cxled_to_port(cxled);
>  	struct range *hpa = &cxled->cxld.hpa_range;
>  	struct cxl_decoder *cxld = &cxled->cxld;
> -	struct device *cxlrd_dev;
>  
>  	while (iter && !is_cxl_root(iter))
>  		iter = to_cxl_port(iter->dev.parent);
> @@ -3217,9 +3232,8 @@ 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) {
> +	cxld = cxl_port_find_switch_decoder(iter, hpa);
> +	if (!cxld) {
>  		dev_err(cxlmd->dev.parent,
>  			"%s:%s no CXL window for range %#llx:%#llx\n",
>  			dev_name(&cxlmd->dev), dev_name(&cxld->dev),
> @@ -3227,16 +3241,9 @@ 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(&cxld->dev);
>  }
>  
>  static int match_region_by_range(struct device *dev, void *data)