linux-kernel - [PATCH v2] iommu/amd: Enhance "Completion-wait Time-out" error message

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Message-ID: <20251024061041.707-1-dheerajkumar.srivastava@amd.com>
Date: Fri, 24 Oct 2025 11:40:41 +0530
From: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@....com>
To: <joro@...tes.org>, <will@...nel.org>, <robin.murphy@....com>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
CC: <suravee.suthikulpanit@....com>, <Vasant.Hegde@....com>,
	<Santosh.Shukla@....com>, Dheeraj Kumar Srivastava
	<dheerajkumar.srivastava@....com>
Subject: [PATCH v2] iommu/amd: Enhance "Completion-wait Time-out" error message

Current IOMMU driver prints "Completion-wait Time-out" error message with
insufficient information to further debug the issue.

Enhancing the error message as following:
1. Log IOMMU PCI device ID in the error message.
2. Also dump the command immediately preceding the COMPLETION_WAIT
   command to identify what the IOMMU is waiting for.

Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@....com>
---
Changes since v1:
-> Remove the code that dumps the entire command buffer on each
   completion wait timeout when amd_iommu_dump=1. [Suravee and Vasant]

 drivers/iommu/amd/amd_iommu_types.h |  2 ++
 drivers/iommu/amd/iommu.c           | 18 +++++++++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/amd/amd_iommu_types.h b/drivers/iommu/amd/amd_iommu_types.h
index 95f63c5f6159..d495e5e4a9a2 100644
--- a/drivers/iommu/amd/amd_iommu_types.h
+++ b/drivers/iommu/amd/amd_iommu_types.h
@@ -247,6 +247,8 @@
 #define CMD_BUFFER_ENTRIES 512
 #define MMIO_CMD_SIZE_SHIFT 56
 #define MMIO_CMD_SIZE_512 (0x9ULL << MMIO_CMD_SIZE_SHIFT)
+#define MMIO_CMD_TAIL_MASK	GENMASK_ULL(18, 4)
+#define MMIO_CMD_BUFFER_TAIL(x)	FIELD_GET(MMIO_CMD_TAIL_MASK, (x))
 
 /* constants for event buffer handling */
 #define EVT_BUFFER_SIZE		8192 /* 512 entries */
diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
index eb348c63a8d0..89f7f6b01cad 100644
--- a/drivers/iommu/amd/iommu.c
+++ b/drivers/iommu/amd/iommu.c
@@ -1158,6 +1158,7 @@ irqreturn_t amd_iommu_int_handler(int irq, void *data)
 
 static int wait_on_sem(struct amd_iommu *iommu, u64 data)
 {
+	struct iommu_cmd *cmd;
 	int i = 0;
 
 	while (*iommu->cmd_sem != data && i < LOOP_TIMEOUT) {
@@ -1166,7 +1167,22 @@ static int wait_on_sem(struct amd_iommu *iommu, u64 data)
 	}
 
 	if (i == LOOP_TIMEOUT) {
-		pr_alert("Completion-Wait loop timed out\n");
+		int head, tail;
+
+		head = readl(iommu->mmio_base + MMIO_CMD_HEAD_OFFSET);
+		tail = readl(iommu->mmio_base + MMIO_CMD_TAIL_OFFSET);
+
+		pr_alert("IOMMU %04x:%02x:%02x.%01x: Completion-Wait loop timed out\n",
+			 iommu->pci_seg->id, PCI_BUS_NUM(iommu->devid),
+			 PCI_SLOT(iommu->devid), PCI_FUNC(iommu->devid));
+		/*
+		 * On command buffer completion timeout, step back by 2 commands
+		 * to locate the actual command that is causing the issue.
+		 */
+		tail = (MMIO_CMD_BUFFER_TAIL(tail) - 2) & (CMD_BUFFER_ENTRIES - 1);
+		cmd = (struct iommu_cmd *)(iommu->cmd_buf + tail * sizeof(*cmd));
+		dump_command(iommu_virt_to_phys(cmd));
+
 		return -EIO;
 	}
 
-- 
2.25.1