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Message-ID: <BN9PR11MB527663460076DA4BC8D34D3F8CBA9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Thu, 30 Jun 2022 09:57:28 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Robin Murphy <robin.murphy@....com>,
Nicolin Chen <nicolinc@...dia.com>,
Yong Wu <yong.wu@...iatek.com>
CC: Jason Gunthorpe <jgg@...dia.com>,
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<linux-mediatek@...ts.infradead.org>
Subject: RE: [PATCH v3 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain
and device/group
> From: Robin Murphy <robin.murphy@....com>
> Sent: Thursday, June 30, 2022 4:22 PM
>
> On 2022-06-29 20:47, Nicolin Chen wrote:
> > On Fri, Jun 24, 2022 at 03:19:43PM -0300, Jason Gunthorpe wrote:
> >> On Fri, Jun 24, 2022 at 06:35:49PM +0800, Yong Wu wrote:
> >>
> >>>>> 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).
> >>
> >> What part of the iommu framework is trying to attach a domain and
> >> wants to see success when the domain was not actually attached ?
> >>
> >>>> What prevent this driver from being used in VFIO context?
> >>>
> >>> Nothing prevent this. Just I didn't test.
> >>
> >> This is why it is wrong to return success here.
> >
> > Hi Yong, would you or someone you know be able to confirm whether
> > this "return 0" is still a must or not?
>
> From memory, it is unfortunately required, due to this driver being in
> the rare position of having to support multiple devices in a single
> address space on 32-bit ARM. Since the old ARM DMA code doesn't
> understand groups, the driver sets up its own canonical
> dma_iommu_mapping to act like a default domain, but then has to politely
> say "yeah OK" to arm_setup_iommu_dma_ops() for each device so that they
> do all end up with the right DMA ops rather than dying in screaming
> failure (the ARM code's per-device mappings then get leaked, but we
> can't really do any better).
>
> The whole mess disappears in the proper default domain conversion, but
> in the meantime, it's still safe to assume that nobody's doing VFIO with
> embedded display/video codec/etc. blocks that don't even have reset drivers.
>
Probably above is worth a comment in mtk code so we don't need
always dig it out from memory when similar question arises in the
the future. 😊
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