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

Li, Ming4
2024-09-17T03:31:46Z

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@amd.com wrote:
>>> From: Alejandro Lucero <alucerop@amd.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@amd.com>
>>> Co-developed-by: Dan Williams <dan.j.williams@intel.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
>>