Subject: Re: [PATCH v2 4/7] iommu/arm-smmu-v3: Mark ATC invalidate timeouts via lockless bitmap

From: Samiullah Khawaja <[email-masked]>
Date: Thu, 19 Mar 2026 00:08:04 +0000

On Wed, Mar 18, 2026 at 04:23:53PM -0700, Nicolin Chen wrote:
>Hi Sami,
>
>On Wed, Mar 18, 2026 at 10:02:32PM +0000, Samiullah Khawaja wrote:
>> On Tue, Mar 17, 2026 at 12:15:37PM -0700, Nicolin Chen wrote:
>> > @@ -895,9 +898,19 @@ int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu,
>> >
>> > 	/* 5. If we are inserting a CMD_SYNC, we must wait for it to complete */
>> > 	if (sync) {
>> > +		u32 sync_prod;
>> > +
>> > 		llq.prod = queue_inc_prod_n(&llq, n);
>> > +		sync_prod = llq.prod;
>> > +
>> > 		ret = arm_smmu_cmdq_poll_until_sync(smmu, cmdq, &llq);
>> > -		if (ret) {
>> > +		if (test_and_clear_bit(Q_IDX(&llq, sync_prod),
>> > +				       cmdq->atc_sync_timeouts)) {
>>
>> This will not be set if a software timeout (1 second) occurs. Do you
>> know if the ATC timeout of Arm sMMUv3 is less than the software timeout
>> in the driver?
>
>You brought up a good point!
>
>I think ATC timeout follows the PCI Completion Timeout Value in
>"Device Control 2 Register", which is typically set [50us, 50ms]
>but can be set up to [17s, 64s] according to PCI Base spec.

Agreed.
>
>> If not maybe we can handle the software timeout here also as the cmdlist
>> is already known?
>
>I think it's trickier.
>
>If the software times out first at 1s, it means the CMDQ is still
>pending on wait for the completion of ATC invalidation. Then, the
>caller sees -ETIMEOUT and tries to bisect the ATC batch or update
>the STE directly, either of which involves CMDQ. But CMDQ has not
>recovered yet.
>
>Then, in case of a batch, all the reties could timeout again. So,
>it will fail to identify which device is truly broken. This would
>end badly by blindly disabling all the devices in the batch. Also
>the disabling calls require CMDQ too, so they might fail as well.

Yes, looking at VT-d currently and the queue length is 256 and this
spirals out of control quickly.
>
>Thus, partially to answer the question, in case software timeout,
>I am afraid that we can hardly do anything.. :-/

Agreed.

Do you think we can maybe document this somewhere? Maybe add to the
cover letter?
>
>This means I need to set a different return code for ATC timeouts
>v.s. software timeouts.
>
>Also, there is another problem: when PCI CTO finally reaches, the
>GERROR ISR will set atc_sync_timeouts but nobody will clear it..
>So, before calling arm_smmu_cmdq_issue_cmdlist(), we need to make
>sure there is no dirty bit on the bitmap too.

Yes, Just to confirm, do you think this needs to be handled regardless
whether we handle the software timeout for the ATC invalidation?
Basically to cleanup the bit on bitmap.
>Thanks!
>Nicolin