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

[Tian, Kevin]

> From: Robin Murphy <robin.murphy@xxxxxxx>
> 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. 😊