linux-kernel - Re: [PATCH v4] iommu/amd: Enhance "Completion-wait Time-out" error message

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Message-ID: <mmmsb4pp37slzy65ez4z5i7z342zx3vdxbll5axuio2ugdzzbt@i6yurbrb7k2o>
Date: Fri, 7 Nov 2025 14:06:59 +0000
From: Ankit Soni <Ankit.Soni@....com>
To: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@....com>
CC: <joro@...tes.org>, <will@...nel.org>, <robin.murphy@....com>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>,
	<suravee.suthikulpanit@....com>, <Vasant.Hegde@....com>,
	<Santosh.Shukla@....com>
Subject: Re: [PATCH v4] iommu/amd: Enhance "Completion-wait Time-out" error
 message

Hello Dheeraj,

On Wed, Nov 05, 2025 at 01:33:42PM +0530, Dheeraj Kumar Srivastava wrote:
> 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. With "amd_iommu_dump=1" kernel command line option, dump entire
>    command buffer entries including Head and Tail offset.
> 
> Dump the entire command buffer only on the first 'Completion-wait Time-out'
> to avoid dmesg spam.
> 
> Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@....com>
> ---
> Changes since v3:
> -> Dump the entire command buffer only on the first 'Completion-wait Time-out'
>    when amd_iommu_dump=1, instead of dumping it on every occurrence.
> 
>  drivers/iommu/amd/amd_iommu_types.h |  4 ++++
>  drivers/iommu/amd/iommu.c           | 28 +++++++++++++++++++++++++++-
>  2 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/amd/amd_iommu_types.h b/drivers/iommu/amd/amd_iommu_types.h
> index 95f63c5f6159..7576814f944d 100644
> --- a/drivers/iommu/amd/amd_iommu_types.h
> +++ b/drivers/iommu/amd/amd_iommu_types.h
> @@ -247,6 +247,10 @@
>  #define CMD_BUFFER_ENTRIES 512
>  #define MMIO_CMD_SIZE_SHIFT 56
>  #define MMIO_CMD_SIZE_512 (0x9ULL << MMIO_CMD_SIZE_SHIFT)

It's worth adding comment for MASK 4-18.

> +#define MMIO_CMD_HEAD_MASK	GENMASK_ULL(18, 4)
> +#define MMIO_CMD_BUFFER_HEAD(x) FIELD_GET(MMIO_CMD_HEAD_MASK, (x))
> +#define MMIO_CMD_TAIL_MASK	GENMASK_ULL(18, 4)

Since HEAD and TAIL masks are same, may be, you can use something like
MMIO_CMD_HEAD_TAIL_MASK().

> +#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..abce078d2323 100644
> --- a/drivers/iommu/amd/iommu.c
> +++ b/drivers/iommu/amd/iommu.c
> @@ -1156,6 +1156,25 @@ irqreturn_t amd_iommu_int_handler(int irq, void *data)
>   *
>   ****************************************************************************/
>  
> +static void dump_command_buffer(struct amd_iommu *iommu)
> +{
> +	struct iommu_cmd *cmd;
> +	int head, tail, i;

Since readl returns u32, prefer u32 for head/tail to avoid any surprises
or sign-ext issues.
and similarly use %u in pr_err() below.

> +
> +	head = readl(iommu->mmio_base + MMIO_CMD_HEAD_OFFSET);
> +	tail = readl(iommu->mmio_base + MMIO_CMD_TAIL_OFFSET);
> +
> +	pr_err("CMD Buffer head=%d tail=%d\n", (int)(MMIO_CMD_BUFFER_HEAD(head)),
> +	       (int)(MMIO_CMD_BUFFER_TAIL(tail)));
> +
> +	for (i = 0; i < CMD_BUFFER_ENTRIES; i++) {
> +		cmd = (struct iommu_cmd *)(iommu->cmd_buf + i * sizeof(*cmd));
> +		pr_err("%3d: %08x %08x %08x %08x\n", i, cmd->data[0], cmd->data[1], cmd->data[2],
> +		       cmd->data[3]);
> +	}
> +}
> +
> +

Remove extra line.

Best,
Ankit

>  static int wait_on_sem(struct amd_iommu *iommu, u64 data)
>  {
>  	int i = 0;
> @@ -1166,7 +1185,14 @@ static int wait_on_sem(struct amd_iommu *iommu, u64 data)
>  	}
>  
>  	if (i == LOOP_TIMEOUT) {
> -		pr_alert("Completion-Wait loop timed out\n");
> +
> +		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));
> +
> +		if (amd_iommu_dump)
> +			DO_ONCE_LITE(dump_command_buffer, iommu);
> +
>  		return -EIO;
>  	}
>  
> -- 
> 2.25.1
>