linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue

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Message-ID: <bebc7b1b-e60c-d5e6-812a-63580d7029cc@huawei.com>
Date:   Thu, 5 Aug 2021 11:22:15 +0100
From:   John Garry <john.garry@...wei.com>
To:     <will@...nel.org>, <robin.murphy@....com>
CC:     <joro@...tes.org>, <linux-arm-kernel@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>, <linux-kernel@...r.kernel.org>,
        <linuxarm@...wei.com>
Subject: Re: [PATCH] iommu/arm-smmu-v3: Remove some unneeded init in
 arm_smmu_cmdq_issue_cmdlist()

On 21/06/2021 17:36, John Garry wrote:
> Members of struct "llq" will be zero-inited, apart from member max_n_shift.
> But we write llq.val straight after the init, so it was pointless to zero
> init those other members. As such, separately init member max_n_shift
> only.
> 
> In addition, struct "head" is initialised to "llq" only so that member
> max_n_shift is set. But that member is never referenced for "head", so
> remove any init there.
> 
> Removing these initializations is seen as a small performance optimisation,
> as this code is (very) hot path.
> 

Hi Will,

Any chance you can pick up this small optimisation?

Cheers

> Signed-off-by: John Garry <john.garry@...wei.com>
> 
> 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 54b2f27b81d4..8a8ad49bb7fd 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -727,11 +727,11 @@ static int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu,
>   	unsigned long flags;
>   	bool owner;
>   	struct arm_smmu_cmdq *cmdq = &smmu->cmdq;
> -	struct arm_smmu_ll_queue llq = {
> -		.max_n_shift = cmdq->q.llq.max_n_shift,
> -	}, head = llq;
> +	struct arm_smmu_ll_queue llq, head;
>   	int ret = 0;
>   
> +	llq.max_n_shift = cmdq->q.llq.max_n_shift;
> +
>   	/* 1. Allocate some space in the queue */
>   	local_irq_save(flags);
>   	llq.val = READ_ONCE(cmdq->q.llq.val);
>