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Date: Tue, 25 Nov 2025 17:19:16 +0000
From: Will Deacon <will@kernel.org>
To: Jacob Pan <jacob.pan@linux.microsoft.com>
Cc: linux-kernel@vger.kernel.org,
	"iommu@lists.linux.dev" <iommu@lists.linux.dev>,
	Joerg Roedel <joro@8bytes.org>, Mostafa Saleh <smostafa@google.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Nicolin Chen <nicolinc@nvidia.com>,
	Zhang Yu <zhangyu1@linux.microsoft.com>,
	Jean Philippe-Brucker <jean-philippe@linaro.org>,
	Alexander Grest <Alexander.Grest@microsoft.com>
Subject: Re: [PATCH v4 1/2] iommu/arm-smmu-v3: Fix CMDQ timeout warning
Message-ID: <aSXlFDAa672dZGVm@willie-the-truck>
References: <20251114171718.42215-1-jacob.pan@linux.microsoft.com>
 <20251114171718.42215-2-jacob.pan@linux.microsoft.com>
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In-Reply-To: <20251114171718.42215-2-jacob.pan@linux.microsoft.com>

On Fri, Nov 14, 2025 at 09:17:17AM -0800, 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.
> 
> The existing arm_smmu_cmdq_poll_until_not_full() doesn't fit efficiently
> nor ideally to the only caller arm_smmu_cmdq_issue_cmdlist():
>  - It uses a new timer at every single call, which fails to limit to the
>    preset ARM_SMMU_POLL_TIMEOUT_US per issue.
> - It has a redundant internal queue_full(), which doesn't detect whether
>    there is a enough space for number of n commands.
> 
> This patch polls for the availability of exact space instead of full and
> emit timeout warning accordingly.
> 
> Fixes: 587e6c10a7ce ("iommu/arm-smmu-v3: Reduce contention during command-queue insertion")
> Co-developed-by: Yu Zhang <zhangyu1@linux.microsoft.com>
> Signed-off-by: Yu Zhang <zhangyu1@linux.microsoft.com>
> Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>

I'm assuming you're seeing problems with an emulated command queue? Any
chance you could make that bigger?

> @@ -804,12 +794,13 @@ int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu,
>  	local_irq_save(flags);
>  	llq.val = READ_ONCE(cmdq->q.llq.val);
>  	do {
> +		struct arm_smmu_queue_poll qp;
>  		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");
> +			arm_smmu_cmdq_poll(smmu, cmdq, &llq, &qp);

Isn't this broken for wfe-based polling? The SMMU only generates the
wake-up event when the queue becomes non-full.

Will