Re: [PATCH 1/2] iommu/arm-smmu-v3: Fix CMDQ timeout warning

[Nicolin Chen <nicolinc@nvidia.com>]

On Wed, Sep 24, 2025 at 10:54:37AM -0700, Jacob Pan wrote:
> While polling for n spaces in the cmdq, the current code instead checks
> if the queue is full. If the queue is almost full but not enough space
> (<n), then the CMDQ timeout warning is never triggered even if the
> polling has exceeded timeout limit.

This does sound like an issue that is missing a warning print.

> This patch polls for the availability of exact space instead of full and
> emit timeout warning accordingly.

And the solution sounds plausible as well.

> @@ -806,10 +769,28 @@ int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu,
>  	do {
>  		u64 old;
>  
> +		queue_poll_init(smmu, &qp);
>  		while (!queue_has_space(&llq, n + sync)) {
>  			local_irq_restore(flags);
> -			if (arm_smmu_cmdq_poll_until_not_full(smmu, cmdq, &llq))
> -				dev_err_ratelimited(smmu->dev, "CMDQ timeout\n");
> +			/*
> +			 * Try to update our copy of cons by grabbing exclusive cmdq access. If
> +			 * that fails, spin until somebody else updates it for us.
> +			 */
> +			if (arm_smmu_cmdq_exclusive_trylock_irqsave(cmdq, flags)) {
> +				WRITE_ONCE(cmdq->q.llq.cons, readl_relaxed(cmdq->q.cons_reg));
> +				arm_smmu_cmdq_exclusive_unlock_irqrestore(cmdq, flags);
> +				llq.val = READ_ONCE(cmdq->q.llq.val);
> +				local_irq_save(flags);
> +				continue;
> +			}
> +
> +			ret = queue_poll(&qp);
> +			if (ret == -ETIMEDOUT) {
> +				dev_err_ratelimited(smmu->dev, "CPU %d CMDQ Timeout, C: %08x, CW:%x P: 0x%08x PW: %x cmdq.lock 0x%x\n",
> +						    smp_processor_id(), Q_IDX(&llq, llq.cons), Q_WRP(&llq, llq.cons), Q_IDX(&llq, llq.prod), Q_WRP(&llq, llq.prod), atomic_read(&cmdq->lock));
> +				queue_poll_init(smmu, &qp);
> +			}
> +			llq.val = READ_ONCE(cmdq->q.llq.val);
>  			local_irq_save(flags);

But, couldn't we write a new arm_smmu_cmdq_poll_until_enough_space()
simply replacing arm_smmu_cmdq_exclusive_unlock_irqrestore()?

This whole unwrapped piece is really not easy to read :(

Also, the new error message has too much debugging info, which could
be trimmed away, IMHO. Though kernel coding now does allow a higher
limit per line, that 200-ish-character line is a bit overdone :-/

Nicolin