linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: Stop pre-zeroing batch commands in arm_smmu_atc_inv

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Message-ID: <23f1cc66-2172-9534-53ef-3d9b2db26bda@huawei.com>
Date:   Tue, 17 Aug 2021 12:53:52 +0100
From:   John Garry <john.garry@...wei.com>
To:     Zhen Lei <thunder.leizhen@...wei.com>,
        Will Deacon <will@...nel.org>,
        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@...r.kernel.org>
Subject: Re: [PATCH] iommu/arm-smmu-v3: Stop pre-zeroing batch commands in
 arm_smmu_atc_inv_master()

On 17/08/2021 12:34, Zhen Lei wrote:
> Pre-zeroing the batched commands structure is inefficient, as individual
> commands are zeroed later in arm_smmu_cmdq_build_cmd(). Therefore, only
> the member 'num' needs to be initialized to 0.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>

Reviewed-by: John Garry <john.garry@...wei.com>

> ---
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> 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 50a9db5bac466c7..e6882ae81fd08f6 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -1776,10 +1776,11 @@ static int arm_smmu_atc_inv_master(struct arm_smmu_master *master)
>   {
>   	int i;
>   	struct arm_smmu_cmdq_ent cmd;
> -	struct arm_smmu_cmdq_batch cmds = {};
> +	struct arm_smmu_cmdq_batch cmds;
>   
>   	arm_smmu_atc_inv_to_cmd(0, 0, 0, &cmd);
>   
> +	cmds.num = 0;

We prob should have added a comment why we do this (and at the other 
sites). I think Robin said something similar in another patch.

>   	for (i = 0; i < master->num_streams; i++) {
>   		cmd.atc.sid = master->streams[i].id;
>   		arm_smmu_cmdq_batch_add(master->smmu, &cmds, &cmd);
>