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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 >>>> Co-developed-by: Dan Williams >>>> --- >>>>   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 >>>>   #include >>>>   #include >>>> +#include >>>>   #include >>>>   #include >>>>   #include >>>> @@ -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