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Date:   Tue, 26 Jan 2021 22:23:06 +0000
From:   Will Deacon <will@kernel.org>
To:     Yong Wu <yong.wu@mediatek.com>
Cc:     Joerg Roedel <joro@8bytes.org>, Rob Herring <robh+dt@kernel.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Evan Green <evgreen@chromium.org>,
        Tomasz Figa <tfiga@google.com>,
        linux-mediatek@lists.infradead.org, srv_heupstream@mediatek.com,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        iommu@lists.linux-foundation.org, youlin.pei@mediatek.com,
        Nicolas Boichat <drinkcat@chromium.org>, anan.sun@mediatek.com,
        chao.hao@mediatek.com
Subject: Re: [PATCH v6 07/33] iommu: Avoid reallocate default domain for a
 group
Message-ID: <20210126222305.GB30460@willie-the-truck>
References: <20210111111914.22211-1-yong.wu@mediatek.com>
 <20210111111914.22211-8-yong.wu@mediatek.com>
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On Mon, Jan 11, 2021 at 07:18:48PM +0800, Yong Wu wrote:
> If group->default_domain exists, avoid reallocate it.
> 
> In some iommu drivers, there may be several devices share a group. Avoid
> realloc the default domain for this case.
> 
> Signed-off-by: Yong Wu <yong.wu@mediatek.com>
> ---
>  drivers/iommu/iommu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 3d099a31ddca..f4b87e6abe80 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -266,7 +266,8 @@ int iommu_probe_device(struct device *dev)
>  	 * support default domains, so the return value is not yet
>  	 * checked.
>  	 */
> -	iommu_alloc_default_domain(group, dev);
> +	if (!group->default_domain)
> +		iommu_alloc_default_domain(group, dev);

I don't really get what this achieves, since iommu_alloc_default_domain()
looks like this:

static int iommu_alloc_default_domain(struct iommu_group *group,
				      struct device *dev)
{
	unsigned int type;

	if (group->default_domain)
		return 0;

	...

in which case, it should be fine?

Will