> 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. 😊
