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

Alejandro Lucero Palau
2024-09-16T13:25:40Z

On 9/13/24 18:43, Jonathan Cameron wrote:
> On Sat, 7 Sep 2024 09:18:26 +0100
> 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.
> This dance is nasty and there is no real guarantee it will even help.

With all due respect, we know Dan's dancing credentials. Which are
yours?  ... :-)

So, I found this when testing. The driver calls devm_cxl_add_memdev and
then cxl_acquire_endpoint, and the endpoint is not there yet.

Interestingly, I did not suffer it initially, so I do not know what
makes it trigger, but I have just tested it again and if I remove the
iteration with the timeout, the calls fails (it seems it does
deterministically). This is with a testing/developing VM with no other
main processing at the time of driver binding, and with 8 cores
available. Of course, if the driver does the second call without
interruption, it is unlikely another core can handle the bus
notification and create the endpoint faster. But I wonder why this is
(was) not always happening.

I agree the timeout could not be enough in some situations. I think it
all depends on when bus_notify is invoked, but I have not dug deeper.

>
> 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?

You put it in a way that makes me miserable ... but maybe you are right,
since there is no certainty it will be done after the timeout, but it is
a full second ...

I will dig a bit ...

Thanks

>
>> + */
>> + 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