linux-kernel - Re: [PATCH RFC 2/8] iommu/arm-smmu-v3: Add and use static helper function arm_smmu_cmdq_issue_cmd_with

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Date:   Tue, 10 Aug 2021 19:24:54 +0100
From:   Will Deacon <will@...nel.org>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 2/8] iommu/arm-smmu-v3: Add and use static helper
 function arm_smmu_cmdq_issue_cmd_with_sync()

On Sat, Jun 26, 2021 at 07:01:24PM +0800, Zhen Lei wrote:
> The obvious key to the performance optimization of commit 587e6c10a7ce
> ("iommu/arm-smmu-v3: Reduce contention during command-queue insertion") is
> to allow multiple cores to insert commands in parallel after a brief mutex
> contention.
> 
> Obviously, inserting as many commands at a time as possible can reduce the
> number of times the mutex contention participates, thereby improving the
> overall performance. At least it reduces the number of calls to function
> arm_smmu_cmdq_issue_cmdlist().
> 
> Therefore, function arm_smmu_cmdq_issue_cmd_with_sync() is added to insert
> the 'cmd+sync' commands at a time.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 33 +++++++++++++--------
>  1 file changed, 21 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index 2433d3c29b49ff2..a5361153ca1d6a4 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -858,11 +858,25 @@ static int arm_smmu_cmdq_issue_cmd(struct arm_smmu_device *smmu,
>  	return arm_smmu_cmdq_issue_cmdlist(smmu, cmd, 1, false);
>  }
>  
> -static int arm_smmu_cmdq_issue_sync(struct arm_smmu_device *smmu)
> +static int __maybe_unused arm_smmu_cmdq_issue_sync(struct arm_smmu_device *smmu)
>  {
>  	return arm_smmu_cmdq_issue_cmdlist(smmu, NULL, 0, true);
>  }
>  
> +static int arm_smmu_cmdq_issue_cmd_with_sync(struct arm_smmu_device *smmu,
> +					     struct arm_smmu_cmdq_ent *ent)
> +{
> +	u64 cmd[CMDQ_ENT_DWORDS];
> +
> +	if (arm_smmu_cmdq_build_cmd(cmd, ent)) {
> +		dev_warn(smmu->dev, "ignoring unknown CMDQ opcode 0x%x\n",
> +			 ent->opcode);
> +		return -EINVAL;
> +	}
> +
> +	return arm_smmu_cmdq_issue_cmdlist(smmu, cmd, 1, true);
> +}

This function is almost identical to arm_smmu_cmdq_issue_cmd(). How about
moving the guts out into a helper:

	static int __arm_smmu_cmdq_issue_cmd(struct arm_smmu_device *smmu,
					     struct arm_smmu_cmdq_ent *ent,
					     bool sync);

and then having arm_smmu_cmdq_issue_cmd_with_sync() and
arm_smmu_cmdq_issue_cmd() wrap that?

Will