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Message-ID: <20240913184319.00000440@Huawei.com>
Date: Fri, 13 Sep 2024 18:43:19 +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>, <martin.habets@...inx.com>,
<edward.cree@....com>, <davem@...emloft.net>, <kuba@...nel.org>,
<pabeni@...hat.com>, <edumazet@...gle.com>, Alejandro Lucero
<alucerop@....com>
Subject: Re: [PATCH v3 10/20] cxl: indicate probe deferral
On Sat, 7 Sep 2024 09:18:26 +0100
alejandro.lucero-palau@....com wrote:
> From: Alejandro Lucero <alucerop@....com>
>
> 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/168592155270.1948938.11536845108449547920.stgit@dwillia2-xfh.jf.intel.com/
>
> Signed-off-by: Alejandro Lucero <alucerop@....com>
> Co-developed-by: Dan Williams <dan.j.williams@...el.com>
> ---
> 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.
This dance is nasty and there is no real guarantee it will even help.
We need a better solution. I'm not quite sure on what it is though.
Is there any precedence for similar 'wait a bit and hope'
in the kernel?
> + */
> + 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
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