linux-kernel - Re: [PATCH v2 1/2] iommu/arm-smmu-v3: Fix CMDQ timeout warning

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Message-ID: <aQPptXsqzt6kJS7f@Asurada-Nvidia>
Date: Thu, 30 Oct 2025 15:41:57 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jacob Pan <jacob.pan@...ux.microsoft.com>
CC: <linux-kernel@...r.kernel.org>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, Will Deacon <will@...nel.org>, Joerg Roedel
	<joro@...tes.org>, Mostafa Saleh <smostafa@...gle.com>, Jason Gunthorpe
	<jgg@...dia.com>, Robin Murphy <robin.murphy@....com>, Zhang Yu
	<zhangyu1@...ux.microsoft.com>, Jean Philippe-Brucker
	<jean-philippe@...aro.org>, Alexander Grest <Alexander.Grest@...rosoft.com>
Subject: Re: [PATCH v2 1/2] iommu/arm-smmu-v3: Fix CMDQ timeout warning

On Mon, Oct 20, 2025 at 03:43:52PM -0700, Jacob Pan wrote:
> @@ -785,10 +748,33 @@ static 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)) {
> +			unsigned long iflags;
> +
>  			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, iflags)) {
> +				WRITE_ONCE(cmdq->q.llq.cons, readl_relaxed(cmdq->q.cons_reg));
> +				arm_smmu_cmdq_exclusive_unlock_irqrestore(cmdq, iflags);
> +				llq.val = READ_ONCE(cmdq->q.llq.val);
> +				local_irq_save(flags);

I don't quite get the reason why it moves queue_poll_init() and
add local_irq_save(). It's quite different than what the driver
has, so it's nicer to explain in the commit message at least.

I still feel that we could just replace the _until_not_full()
with a _until_has_space()?

Nicolin