Re: [PATCH v3 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group

From: Nicolin Chen <nicolinc@nvidia.com>
To: Baolu Lu <baolu.lu@linux.intel.com>, <yong.wu@mediatek.com>
Cc: <joro@8bytes.org>, <will@kernel.org>, <marcan@marcan.st>,
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Subject: Re: [PATCH v3 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
Date: Thu, 23 Jun 2022 19:44:00 -0700	[thread overview]
Message-ID: <YrUk8IINqDEZLfIa@Asurada-Nvidia> (raw)
In-Reply-To: <270eec00-8aee-2288-4069-d604e6da2925@linux.intel.com>

On Fri, Jun 24, 2022 at 09:35:49AM +0800, Baolu Lu wrote:
> External email: Use caution opening links or attachments
> 
> 
> On 2022/6/24 04:00, Nicolin Chen wrote:
> > diff --git a/drivers/iommu/mtk_iommu_v1.c b/drivers/iommu/mtk_iommu_v1.c
> > index e1cb51b9866c..5386d889429d 100644
> > --- a/drivers/iommu/mtk_iommu_v1.c
> > +++ b/drivers/iommu/mtk_iommu_v1.c
> > @@ -304,7 +304,7 @@ static int mtk_iommu_v1_attach_device(struct iommu_domain *domain, struct device
> >       /* Only allow the domain created internally. */
> >       mtk_mapping = data->mapping;
> >       if (mtk_mapping->domain != domain)
> > -             return 0;
> > +             return -EMEDIUMTYPE;
> > 
> >       if (!data->m4u_dom) {
> >               data->m4u_dom = dom;
> 
> This change looks odd. It turns the return value from success to
> failure. Is it a bug? If so, it should go through a separated fix patch.

Makes sense.

I read the commit log of the original change:
https://lore.kernel.org/r/1589530123-30240-1-git-send-email-yong.wu@mediatek.com

It doesn't seem to allow devices to get attached to different
domains other than the shared mapping->domain, created in the
in the mtk_iommu_probe_device(). So it looks like returning 0
is intentional. Though I am still very confused by this return
value here, I doubt it has ever been used in a VFIO context.

Young, would you please give us some input?

Overall, I feel it's better to play it safe here by dropping
this part. If we later confirm there is a need to fix it, we
will do that in a separate patch anyway.

Thanks
Nic