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

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Message-ID: <eb0624df-3cae-46ec-9f7e-b45bea46a345@amd.com>
Date: Wed, 5 Nov 2025 12:13:48 +0530
From: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@....com>
To: Jörg Rödel <joro@...tes.org>
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 Joerg,

On 10/27/2025 6:19 PM, Jörg Rödel wrote:
> 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.

In the early boot process, the command buffer is used but 
iommu->dev->dev is not set, which can lead to a kernel crash with dev_err.

Thanks
Dheeraj

> 
>> +		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