linux-kernel - [PATCH v2] cxl/core/port: defer endpoint probes when ACPI likely hasn't finished

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Message-ID: <20241004212504.1246-1-gourry@gourry.net>
Date: Fri,  4 Oct 2024 17:25:04 -0400
From: Gregory Price <gourry@...rry.net>
To: linux-cxl@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	dan.j.williams@...el.com,
	Jonathan.Cameron@...wei.com,
	dave.jiang@...el.com,
	alison.schofield@...el.com,
	vishal.l.verma@...el.com,
	ira.weiny@...el.com,
	rrichter@....com,
	terry.bowman@....com,
	dave@...olabs.net
Subject: [PATCH v2] cxl/core/port: defer endpoint probes when ACPI likely hasn't finished

In cxl_acpi_probe, we add dports and uports to host bridges iteratively:
- bus_for_each_dev(adev->dev.bus, NULL, root_port, add_host_bridge_dport);
- bus_for_each_dev(adev->dev.bus, NULL, root_port, add_host_bridge_uport);

Simultaneously, as ports are probed, memdev endpoints can also be
probed. This creates a race condition, where an endpoint can perceive
its path to the root being broken in devm_cxl_enumerate_ports.

The memdev/endpoint probe will see a heirarchy that may look like:
    mem1
      parent => 0000:c1:00.0
        parent => 0000:c0:01.1
	  parent->parent => NULL

This results in find_cxl_port() returning NULL (since the port hasn't
been associated with the host bridge yet), and add_port_attach_ep
fails because the grandparent's grandparent is NULL.

When the latter condition is detected, the comments suggest:
    /*
     * The iteration reached the topology root without finding the
     * CXL-root 'cxl_port' on a previous iteration, fail for now to
     * be re-probed after platform driver attaches.
     */

This case results in an -ENXIO; however, a re-probe never occurs. Change
this return condition to -EPROBE_DEFER to explicitly cause a reprobe.

(v2: additional debug information)

Signed-off-by: Gregory Price <gourry@...rry.net>
---
 drivers/cxl/core/port.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
index 1d5007e3795a..d6bebf70d142 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);
-- 
2.43.0