linux-kernel - Re: [PATCH v2 3/4] iommu/mediatek: Use writel for TLB range invalidation

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Message-ID: <20191011162950.yg4o77mlaicacne5@willie-the-truck>
Date:   Fri, 11 Oct 2019 17:29:51 +0100
From:   Will Deacon <will@...nel.org>
To:     Yong Wu <yong.wu@...iatek.com>
Cc:     Matthias Brugger <matthias.bgg@...il.com>,
        Joerg Roedel <joro@...tes.org>,
        Will Deacon <will.deacon@....com>,
        Evan Green <evgreen@...omium.org>,
        Robin Murphy <robin.murphy@....com>,
        Tomasz Figa <tfiga@...gle.com>,
        linux-mediatek@...ts.infradead.org, srv_heupstream@...iatek.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, youlin.pei@...iatek.com,
        Nicolas Boichat <drinkcat@...omium.org>, anan.sun@...iatek.com,
        cui.zhang@...iatek.com, chao.hao@...iatek.com
Subject: Re: [PATCH v2 3/4] iommu/mediatek: Use writel for TLB range
 invalidation

On Wed, Oct 09, 2019 at 09:19:02PM +0800, Yong Wu wrote:
> Use writel for the register F_MMU_INV_RANGE which is for triggering the
> HW work. We expect all the setting(iova_start/iova_end...) have already
> been finished before F_MMU_INV_RANGE.
> 
> Signed-off-by: Anan.Sun <anan.sun@...iatek.com>
> Signed-off-by: Yong Wu <yong.wu@...iatek.com>
> ---
> This is a improvement rather than fixing a issue.
> ---
>  drivers/iommu/mtk_iommu.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/mtk_iommu.c b/drivers/iommu/mtk_iommu.c
> index 24a13a6..607f92c 100644
> --- a/drivers/iommu/mtk_iommu.c
> +++ b/drivers/iommu/mtk_iommu.c
> @@ -187,8 +187,7 @@ static void mtk_iommu_tlb_add_flush(unsigned long iova, size_t size,
>  		writel_relaxed(iova, data->base + REG_MMU_INVLD_START_A);
>  		writel_relaxed(iova + size - 1,
>  			       data->base + REG_MMU_INVLD_END_A);
> -		writel_relaxed(F_MMU_INV_RANGE,
> -			       data->base + REG_MMU_INVALIDATE);
> +		writel(F_MMU_INV_RANGE, data->base + REG_MMU_INVALIDATE);

I don't understand this change.

Why is it an "improvement" and which accesses are you ordering with the
writel?

Will