netdev - Re: [PATCH v18 08/20] cx/memdev: Indicate probe deferral

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Message-ID: <20250918121914.00000af9@huawei.com>
Date: Thu, 18 Sep 2025 12:19:14 +0100
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: <alejandro.lucero-palau@....com>
CC: <linux-cxl@...r.kernel.org>, <netdev@...r.kernel.org>,
	<dan.j.williams@...el.com>, <edward.cree@....com>, <davem@...emloft.net>,
	<kuba@...nel.org>, <pabeni@...hat.com>, <edumazet@...gle.com>,
	<dave.jiang@...el.com>, Alejandro Lucero <alucerop@....com>
Subject: Re: [PATCH v18 08/20] cx/memdev: Indicate probe deferral

On Thu, 18 Sep 2025 10:17:34 +0100
alejandro.lucero-palau@....com wrote:

> From: Alejandro Lucero <alucerop@....com>
> 
> The first step for a CXL accelerator driver that wants to establish new
> CXL.mem regions is to register a 'struct cxl_memdev'. That kicks off
> cxl_mem_probe() to enumerate all 'struct cxl_port' instances in the
> topology up to the root.
> 
> If the port driver has not attached yet the expectation is that the
> driver waits until that link is established. The common cxl_pci driver
> has reason to keep the 'struct cxl_memdev' device attached to the bus
> until the root driver attaches. An accelerator may want to instead defer
> probing until CXL resources can be acquired.
> 
> Use the @endpoint attribute of a 'struct cxl_memdev' to convey when a
> accelerator driver probing should be deferred vs failed. Provide that
> indication via a new cxl_acquire_endpoint() API that can retrieve the
> probe status of the memdev.
> 
> Signed-off-by: Alejandro Lucero <alucerop@....com>
In general this seems ok to me.  A few minor comments inline.

> ---
>  drivers/cxl/core/memdev.c | 42 +++++++++++++++++++++++++++++++++++++++
>  drivers/cxl/core/port.c   |  2 +-
>  drivers/cxl/mem.c         |  7 +++++--
>  include/cxl/cxl.h         |  2 ++
>  4 files changed, 50 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cxl/core/memdev.c b/drivers/cxl/core/memdev.c
> index 3228287bf3f0..10d21996598a 100644
> --- a/drivers/cxl/core/memdev.c
> +++ b/drivers/cxl/core/memdev.c
> @@ -1164,6 +1164,48 @@ struct cxl_memdev *cxl_memdev_alloc(struct cxl_dev_state *cxlds,
>  }
>  EXPORT_SYMBOL_NS_GPL(cxl_memdev_alloc, "CXL");
>  
> +/*

Given this is exported and expected to be used by drivers,
it should have full kernel-doc.

> + * Try to get a locked reference on a memdev's CXL port topology
> + * connection. Be careful to observe when cxl_mem_probe() has deposited

I'd describe what that 'connection' means here.  I think it means holding
both the endpoint and cxlmd locks.

> + * a probe deferral awaiting the arrival of the CXL root driver.
> + */
> +struct cxl_port *cxl_acquire_endpoint(struct cxl_memdev *cxlmd)
> +{
> +	struct cxl_port *endpoint;
> +	int rc = -ENXIO;
> +
> +	device_lock(&cxlmd->dev);
> +
> +	endpoint = cxlmd->endpoint;
> +	if (!endpoint)
> +		goto err;
> +
> +	if (IS_ERR(endpoint)) {
> +		rc = PTR_ERR(endpoint);
> +		goto err;
> +	}
> +
> +	device_lock(&endpoint->dev);
> +	if (!endpoint->dev.driver)
> +		goto err_endpoint;
> +
> +	return endpoint;
> +
> +err_endpoint:
> +	device_unlock(&endpoint->dev);
> +err:
> +	device_unlock(&cxlmd->dev);
> +	return ERR_PTR(rc);
> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_acquire_endpoint, "CXL");
> +
> +void cxl_release_endpoint(struct cxl_memdev *cxlmd, struct cxl_port *endpoint)

This is a slightly odd pattern for getting a singleton thing.
We more normally see this sort of release with multi parameter
when there is equivalent of more than one 'endpoint' per 'cxlmd'

> +{
> +	device_unlock(&endpoint->dev);
> +	device_unlock(&cxlmd->dev);
> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_release_endpoint, "CXL");
> +
>  static void sanitize_teardown_notifier(void *data)
>  {
>  	struct cxl_memdev_state *mds = data;
> diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
> index 240c3c5bcdc8..4c3fecd4c8ea 100644
> --- a/drivers/cxl/core/port.c
> +++ b/drivers/cxl/core/port.c
> @@ -1557,7 +1557,7 @@ static int add_port_attach_ep(struct cxl_memdev *cxlmd,
>  		 */
>  		dev_dbg(&cxlmd->dev, "%s is a root dport\n",
>  			dev_name(dport_dev));
> -		return -ENXIO;
> +		return -EPROBE_DEFER;
>  	}
>  
>  	struct cxl_port *parent_port __free(put_cxl_port) =
> diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> index 9ffee09fcb50..f103e2003add 100644
> --- a/drivers/cxl/mem.c
> +++ b/drivers/cxl/mem.c
> @@ -122,14 +122,17 @@ static int cxl_mem_probe(struct device *dev)
>  		return rc;
>  
>  	rc = devm_cxl_enumerate_ports(cxlmd);
> -	if (rc)
> +	if (rc) {
> +		cxlmd->endpoint = ERR_PTR(rc);
>  		return rc;
> +	}
>  
>  	struct cxl_port *parent_port __free(put_cxl_port) =
>  		cxl_mem_find_port(cxlmd, &dport);
>  	if (!parent_port) {
>  		dev_err(dev, "CXL port topology not found\n");
> -		return -ENXIO;
> +		cxlmd->endpoint = ERR_PTR(-EPROBE_DEFER);
> +		return -EPROBE_DEFER;
>  	}
>  
>  	if (cxl_pmem_size(cxlds) && IS_ENABLED(CONFIG_CXL_PMEM)) {
> diff --git a/include/cxl/cxl.h b/include/cxl/cxl.h
> index 401a59185608..64946e698f5f 100644
> --- a/include/cxl/cxl.h
> +++ b/include/cxl/cxl.h
> @@ -251,4 +251,6 @@ int cxl_set_capacity(struct cxl_dev_state *cxlds, u64 capacity);
>  struct cxl_memdev *devm_cxl_add_memdev(struct device *host,
>  				       struct cxl_dev_state *cxlds,
>  				       const struct cxl_memdev_ops *ops);
> +struct cxl_port *cxl_acquire_endpoint(struct cxl_memdev *cxlmd);
> +void cxl_release_endpoint(struct cxl_memdev *cxlmd, struct cxl_port *endpoint);
>  #endif /* __CXL_CXL_H__ */