linux-kernel - Re: [PATCH v4 1/2] iommu/arm-smmu-v3: Fix CMDQ timeout warning

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Message-ID: <aRd1D5L1bLesPjeG@Asurada-Nvidia>
Date: Fri, 14 Nov 2025 10:29:35 -0800
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 v4 1/2] iommu/arm-smmu-v3: Fix CMDQ timeout warning

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@...ux.microsoft.com>
> Signed-off-by: Yu Zhang <zhangyu1@...ux.microsoft.com>
> Signed-off-by: Jacob Pan <jacob.pan@...ux.microsoft.com>

Reviewed-by: Nicolin Chen <nicolinc@...dia.com>