linux-kernel - Re: [PATCH v6 07/33] iommu: Avoid reallocate default domain for a group

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Date:   Tue, 26 Jan 2021 22:23:06 +0000
From:   Will Deacon <will@...nel.org>
To:     Yong Wu <yong.wu@...iatek.com>
Cc:     Joerg Roedel <joro@...tes.org>, Rob Herring <robh+dt@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Evan Green <evgreen@...omium.org>,
        Tomasz Figa <tfiga@...gle.com>,
        linux-mediatek@...ts.infradead.org, srv_heupstream@...iatek.com,
        devicetree@...r.kernel.org, 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,
        chao.hao@...iatek.com
Subject: Re: [PATCH v6 07/33] iommu: Avoid reallocate default domain for a
 group

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@...iatek.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