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

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Date:   Thu, 23 Jun 2022 22:41:14 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Yong Wu <yong.wu@...iatek.com>
CC:     Baolu Lu <baolu.lu@...ux.intel.com>, <joro@...tes.org>,
        <will@...nel.org>, <marcan@...can.st>, <sven@...npeter.dev>,
        <robin.murphy@....com>, <robdclark@...il.com>,
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        <jean-philippe@...aro.org>, <alex.williamson@...hat.com>,
        <jgg@...dia.com>, <kevin.tian@...el.com>,
        <suravee.suthikulpanit@....com>, <alyssa@...enzweig.io>,
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        <jordan@...micpenguin.net>, <iommu@...ts.linux-foundation.org>,
        <linux-kernel@...r.kernel.org>,
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Subject: Re: [PATCH v3 1/5] iommu: Return -EMEDIUMTYPE for incompatible
 domain and device/group

On Fri, Jun 24, 2022 at 01:38:58PM +0800, Yong Wu 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.
> 
> Thanks for the review:)
> 
> >
> > 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.
> 
> It's not used in VFIO context. "return 0" just satisfy the iommu
> framework to go ahead. and yes, here we only allow the shared "mapping-
> >domain" (All the devices share a domain created internally).
> 
> thus I think we should still keep "return 0" here.

Thanks for the reply. I will just drop the change of this file.