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Message-ID: <fcfa1cc0-0ba6-c267-18b3-3923cb217ee4@amd.com>
Date: Tue, 17 Sep 2024 10:16:25 +0100
From: Alejandro Lucero Palau <alucerop@....com>
To: "Li, Ming4" <ming4.li@...el.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 9/17/24 04:31, Li, Ming4 wrote:
> 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);
>
>
You are right.
>>
>>>> + }
>>>> + } 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?
Right again.
I can see it now.
Thank you!
>
>>>> +
>>>> + 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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