[PATCH] PCI/DOE: Poll DOE Busy bit for up to 1 second in pci_doe_send_req

[Gregory Price]

During initial device probe, the PCI DOE busy bit for some CXL
devices may be left set for a longer period than expected by the
current driver logic. Despite local comments stating DOE Busy is
unlikely to be detected, it appears commonly specifically during
boot when CXL devices are being probed.

This was observed on a single socket AMD platform with 2 CXL memory
expanders attached to the single socket. It was not the case that
concurrent accesses were being made, as validated by monitoring
mailbox commands on the device side.

This behavior has been observed with multiple CXL memory expanders
from different vendors - so it appears unrelated to the model.

In all observed tests, only a small period of the retry window is
actually used - typically only a handful of loop iterations.

Polling on the PCI DOE Busy Bit for (at max) one PCI DOE timeout
interval (1 second), resolves this issues cleanly.

Per PCIe r6.2 sec 6.30.3, the DOE Busy Bit being cleared does not
raise an interrupt, so polling is the best option in this scenario.

Subsqeuent code in doe_statemachine_work and abort paths also wait
for up to 1 PCI DOE timeout interval, so this order of (potential)
additional delay is presumed acceptable.

Suggested-by: Lukas Wunner <lukas@xxxxxxxxx>
Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
---
 drivers/pci/doe.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/doe.c b/drivers/pci/doe.c
index 652d63df9d22..27ba5d281384 100644
--- a/drivers/pci/doe.c
+++ b/drivers/pci/doe.c
@@ -149,14 +149,26 @@ static int pci_doe_send_req(struct pci_doe_mb *doe_mb,
 	size_t length, remainder;
 	u32 val;
 	int i;
+	unsigned long timeout_jiffies;
 
 	/*
 	 * Check the DOE busy bit is not set. If it is set, this could indicate
 	 * someone other than Linux (e.g. firmware) is using the mailbox. Note
 	 * it is expected that firmware and OS will negotiate access rights via
 	 * an, as yet to be defined, method.
+	 *
+	 * Wait up to one PCI_DOE_TIMEOUT period to allow the prior command to
+	 * finish. Otherwise, simply error out as unable to field the request.
+	 *
+	 * PCIe r6.2 sec 6.30.3 states no interrupt is raised when the DOE Busy
+	 * bit is cleared, so polling here is our best option for the moment.
 	 */
-	pci_read_config_dword(pdev, offset + PCI_DOE_STATUS, &val);
+	timeout_jiffies = jiffies + PCI_DOE_TIMEOUT;
+	do {
+		pci_read_config_dword(pdev, offset + PCI_DOE_STATUS, &val);
+	} while (FIELD_GET(PCI_DOE_STATUS_BUSY, val) &&
+		 !time_after(jiffies, timeout_jiffies));
+
 	if (FIELD_GET(PCI_DOE_STATUS_BUSY, val))
 		return -EBUSY;
 
-- 
2.43.0