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Message-ID: <601e5516-3ad9-45d7-90ae-635aac14e371@intel.com>
Date: Tue, 17 Sep 2024 11:31:18 +0800
From: "Li, Ming4" <ming4.li@...el.com>
To: Alejandro Lucero Palau <alucerop@....com>,
<alejandro.lucero-palau@....com>, <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>
Subject: Re: [PATCH v3 10/20] cxl: indicate probe deferral
On 9/16/2024 4:24 PM, Alejandro Lucero Palau wrote:
>
> On 9/10/24 07:37, Li, Ming4 wrote:
>> On 9/7/2024 4:18 PM, 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.
>>> + */
>>> + 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;
>> Can exit directly. not need to hold the lock of cxlmd->dev then break.
>
>
> Not sure if it is safe to do device_unlock twice, but even if so, it looks better to my eyes to get the lock or if not to add another error path.
>
why device_unlock will be called twice? directly return the value of rc like below if the sleep is interrupted.
if (msleep_interruptible(100))
return ERR_PTR(rc);
>
>
>>
>>> + }
>>> + } while (!time_after(jiffies, timeout));
Another issue I noticed is that above loop will not hold the device lock if timeout happened(without msleep interrupted), but below "goto err" will call device_unlock() for the device.
I think below 'if (!endpoint)' can also return the value of rc. Combine above changes, I think the code should be:
do {
......
if (msleep_interruptible(100))
break;
} while (!time_after(jiffies, timeout));
if (!endpoint)
return ERR_PTR(rc);
Does it make more sense?
>>> +
>>> + 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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