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

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Message-ID: <tgugv7ebuxtnp6ou5jnt7od7jmccsipblqityiinhkgratosu6@bxsr73hpiiny>
Date: Mon, 27 Oct 2025 13:49:12 +0100
From: Jörg Rödel <joro@...tes.org>
To: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@....com>
Cc: 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] iommu/amd: Enhance "Completion-wait Time-out" error
 message

Hey Dheeraj,

On Thu, Oct 16, 2025 at 08:38:09PM +0530, Dheeraj Kumar Srivastava wrote:
>  static int wait_on_sem(struct amd_iommu *iommu, u64 data)
>  {
> -	int i = 0;
> +	struct iommu_cmd *cmd;
> +	int i = 0, j;
>  
>  	while (*iommu->cmd_sem != data && i < LOOP_TIMEOUT) {
>  		udelay(1);
> @@ -1166,7 +1167,33 @@ 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));

Better use dev_err(&amd_iommu->dev->dev, ...) here.

> +		if (!amd_iommu_dump) {
> +			/*
> +			 * 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));
> +		} else {
> +			/* Dump entire command buffer along with head and tail indices */
> +			pr_alert("CMD Buffer head=%d tail=%d\n", (int)(MMIO_CMD_BUFFER_HEAD(head)),
> +				 (int)(MMIO_CMD_BUFFER_TAIL(tail)));
> +			for (j = 0; j < CMD_BUFFER_ENTRIES; j++) {
> +				cmd = (struct iommu_cmd *)(iommu->cmd_buf + j * sizeof(*cmd));
> +				pr_err("%3d: %08x %08x %08x %08x\n", j, cmd->data[0], cmd->data[1],
> +				       cmd->data[2], cmd->data[3]);
> +			}
> +		}

I don't think it makes much sense to just print the command before the failed
completion wait. In case of a timeout and amd_iommu_dump == true, just dump the
whole pending command buffer, from head to tail.

Also, please put all the parsing/dumping code into a separate function.

Regards,

	Joerg