Subject: Re: [PATCH v3 10/20] cxl: indicate probe deferral

From: Zhi Wang <[email-masked]>
Date: Thu, 12 Sep 2024 12:19:08 +0300

On Sat, 7 Sep 2024 09:18:26 +0100
<[email-masked]> wrote:

> From: Alejandro Lucero <[email-masked]>
> 

Hi Alejandro:

When working with V2, I noticed that if CONFIG_CXL_MEM=m and cxl_mem.ko
is not loaded, loading the type-2 driver would fail on
cxl_acquire_endpoint(). Not sure if you met the same problem.

Now we are waiting for it to be loaded, it seems not ideal with the
problem.

Thanks,
Zhi.

> The first stop 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 root 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
> 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.
> 
> Based on
> https://lore.kernel.org/linux-cxl/[email-masked]/
> 
> Signed-off-by: Alejandro Lucero <[email-masked]>
> Co-developed-by: Dan Williams <[email-masked]>
> ---
>  drivers/cxl/core/memdev.c | 67
> +++++++++++++++++++++++++++++++++++++++ drivers/cxl/core/port.c   |
> 2 +- drivers/cxl/mem.c         |  4 ++-
>  include/linux/cxl/cxl.h   |  2 ++
>  4 files changed, 73 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cxl/core/memdev.c b/drivers/cxl/core/memdev.c
> index 5f8418620b70..d4406cf3ed32 100644
> --- a/drivers/cxl/core/memdev.c
> +++ b/drivers/cxl/core/memdev.c
> @@ -5,6 +5,7 @@
>  #include <linux/io-64-nonatomic-lo-hi.h>
>  #include <linux/firmware.h>
>  #include <linux/device.h>
> +#include <linux/delay.h>
>  #include <linux/slab.h>
>  #include <linux/idr.h>
>  #include <linux/pci.h>
> @@ -23,6 +24,8 @@ static DECLARE_RWSEM(cxl_memdev_rwsem);
>  static int cxl_mem_major;
>  static DEFINE_IDA(cxl_memdev_ida);
>  
> +static unsigned short endpoint_ready_timeout = HZ;
> +
>  static void cxl_memdev_release(struct device *dev)
>  {
>  	struct cxl_memdev *cxlmd = to_cxl_memdev(dev);
> @@ -1163,6 +1166,70 @@ struct cxl_memdev *devm_cxl_add_memdev(struct
> device *host, }
>  EXPORT_SYMBOL_NS_GPL(devm_cxl_add_memdev, CXL);
>  
> +/*
> + * Try to get a locked reference on a memdev's CXL port topology
> + * connection. Be careful to observe when cxl_mem_probe() has
> deposited
> + * 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;
> +	unsigned long timeout;
> +	int rc = -ENXIO;
> +
> +	/*
> +	 * A memdev creation triggers ports creation through the
> kernel
> +	 * device object model. An endpoint port could not be
> created yet
> +	 * but coming. Wait here for a gentle space of time for
> ensuring
> +	 * and endpoint port not there is due to some error and not
> because
> +	 * the race described.
> +	 *
> +	 * Note this is a similar case this function is implemented
> for, but
> +	 * instead of the race with the root port, this is against
> its own
> +	 * endpoint port.
> +	 */
> +	timeout = jiffies + endpoint_ready_timeout;
> +	do {
> +		device_lock(&cxlmd->dev);
> +		endpoint = cxlmd->endpoint;
> +		if (endpoint)
> +			break;
> +		device_unlock(&cxlmd->dev);
> +		if (msleep_interruptible(100)) {
> +			device_lock(&cxlmd->dev);
> +			break;
> +		}
> +	} while (!time_after(jiffies, timeout));
> +
> +	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(cxl_acquire_endpoint, CXL);
> +
> +void cxl_release_endpoint(struct cxl_memdev *cxlmd, struct cxl_port
> *endpoint) +{
> +	device_unlock(&endpoint->dev);
> +	device_unlock(&cxlmd->dev);
> +}
> +EXPORT_SYMBOL_NS(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 39b20ddd0296..ca2c993faa9c 100644
> --- a/drivers/cxl/core/port.c
> +++ b/drivers/cxl/core/port.c
> @@ -1554,7 +1554,7 @@ 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;
>  	}
>  
>  	parent_port = find_cxl_port(dparent, &parent_dport);
> diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> index 5c7ad230bccb..56fd7a100c2f 100644
> --- a/drivers/cxl/mem.c
> +++ b/drivers/cxl/mem.c
> @@ -145,8 +145,10 @@ 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;
> +	}
>  
>  	parent_port = cxl_mem_find_port(cxlmd, &dport);
>  	if (!parent_port) {
> diff --git a/include/linux/cxl/cxl.h b/include/linux/cxl/cxl.h
> index fc0859f841dc..7e4580fb8659 100644
> --- a/include/linux/cxl/cxl.h
> +++ b/include/linux/cxl/cxl.h
> @@ -57,4 +57,6 @@ int cxl_release_resource(struct cxl_dev_state
> *cxlds, enum cxl_resource type); void cxl_set_media_ready(struct
> cxl_dev_state *cxlds); struct cxl_memdev *devm_cxl_add_memdev(struct
> device *host, struct cxl_dev_state *cxlds);
> +struct cxl_port *cxl_acquire_endpoint(struct cxl_memdev *cxlmd);
> +void cxl_release_endpoint(struct cxl_memdev *cxlmd, struct cxl_port
> *endpoint); #endif