Re: [PATCH v2 08/15] cxl: indicate probe deferral

Jonathan Cameron
2024-08-04T17:41:41Z

On Mon, 15 Jul 2024 18:28:28 +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 defferred vs failed. Provide that
> indication via a new cxl_acquire_endpoint() API that can retrieve the
> probe status of the memdev.
>
> The first consumer of this API is a test driver that excercises the CXL
Spell check.
exercises

> Type-2 flow.
>
> Based on https://lore.kernel.org/linux-cxl/168592149709.1948938.8663425987110396027.stgit@dwillia2-xfh.jf.intel.com/T/#m18497367d2ae38f88e94c06369eaa83fa23e92b2
>
> Signed-off-by: Alejandro Lucero <alucerop@amd.com>
> Co-developed-by: Dan Williams <dan.j.williams@intel.com>
> ---
> drivers/cxl/core/memdev.c | 41 ++++++++++++++++++++++++++++++
> drivers/cxl/core/port.c | 2 +-
> drivers/cxl/mem.c | 7 +++--
> drivers/net/ethernet/sfc/efx_cxl.c | 10 +++++++-
> include/linux/cxl_accel_mem.h | 3 +++
> 5 files changed, 59 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/cxl/core/memdev.c b/drivers/cxl/core/memdev.c
> index b902948b121f..d51c8bfb32e3 100644
> --- a/drivers/cxl/core/memdev.c
> +++ b/drivers/cxl/core/memdev.c
> @@ -1137,6 +1137,47 @@ 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

It might have deposited an error that isn't deferral I think.
I would be careful to make that clear in this comment.

> +*/
> +struct cxl_port *cxl_acquire_endpoint(struct cxl_memdev *cxlmd)
> +{
> + struct cxl_port *endpoint;
> + int rc = -ENXIO;
> +
> + device_lock(&cxlmd->dev);

I'd not really expect an 'acquire endpoint' to exit
in the good path with the cxlmd->dev device lock held.
Perhaps that needs a bit more shouting in the naming of
the function?

> + endpoint = cxlmd->endpoint;
> + 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 d66c6349ed2d..3c6b896c5f65 100644
> --- a/drivers/cxl/core/port.c
> +++ b/drivers/cxl/core/port.c
> @@ -1553,7 +1553,7 @@ 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 f76af75a87b7..383a6f4829d3 100644
> --- a/drivers/cxl/mem.c
> +++ b/drivers/cxl/mem.c
> @@ -145,13 +145,16 @@ 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) {
> dev_err(dev, "CXL port topology not found\n");

Hmm. This seems excessive error print for a deferred path.

> - return -ENXIO;
> + cxlmd->endpoint = ERR_PTR(-EPROBE_DEFER);
> + return -EPROBE_DEFER;
> }
>
> if (resource_size(&cxlds->pmem_res) && IS_ENABLED(CONFIG_CXL_PMEM)) {