On Fri, Jul 27, 2018 at 05:02:37PM +0100, Robin Murphy wrote: > On 27/07/18 15:10, Dmitry Osipenko wrote: > >On Friday, 27 July 2018 12:03:28 MSK Will Deacon wrote: > >>On Fri, Jul 27, 2018 at 10:25:13AM +0200, Joerg Roedel wrote: > >>>On Fri, Jul 27, 2018 at 02:16:18AM +0300, Dmitry Osipenko wrote: > >>>>The proposed solution adds a new option to the base device driver > >>>>structure that allows device drivers to explicitly convey to the drivers > >>>>core that the implicit IOMMU backing for devices must not happen. > >>> > >>>Why is IOMMU mapping a problem for the Tegra GPU driver? > >>> > >>>If we add something like this then it should not be the choice of the > >>>device driver, but of the user and/or the firmware. > >> > >>Agreed, and it would still need somebody to configure an identity domain so > >>that transactions aren't aborted immediately. We currently allow the > >>identity domain to be used by default via a command-line option, so I guess > >>we'd need a way for firmware to request that on a per-device basis. > > > >The IOMMU mapping itself is not a problem, the problem is the management of > >the IOMMU. For Tegra we don't want anything to intrude into the IOMMU > >activities because: > > > >1) GPU HW require additional configuration for the IOMMU usage and dumb > >mapping of the allocations simply doesn't work. > > Generally, that's already handled by the DRM drivers allocating > their own unmanaged domains. The only problem we really need to > solve in that regard is that currently the device DMA ops don't get > updated when moving away from the managed domain. That's been OK for > the VFIO case where the device is bound to a different driver which > we know won't make any explicit DMA API calls, but for the more > general case of IOMMU-aware drivers we could certainly do with a bit > of cooperation between the IOMMU API, DMA API, and arch code to > update the DMA ops dynamically to cope with intermediate subsystems > making DMA API calls on behalf of devices they don't know the > intimate details of. > > >2) Older Tegra generations have a limited resource and capabilities in regards > >to IOMMU usage, allocating IOMMU domain per-device is just impossible for > >example. > > > >3) HW performs context switches and so particular allocations have to be > >assigned to a particular contexts IOMMU domain. > > I understand Qualcomm SoCs have a similar thing too, and AFAICS that > case just doesn't fit into the current API model at all. We need the > IOMMU driver to somehow know about the specific details of which > devices have magic associations with specific contexts, and we > almost certainly need a more expressive interface than > iommu_domain_alloc() to have any hope of reliable results. This is correct for Qualcomm GPUs - The GPU hardware context switching requires a specific context and there are some restrictions around secure contexts as well. We don't really care if the DMA attaches to a context just as long as it doesn't attach to the one(s) we care about. Perhaps a "valid context" mask would work in from the DT or the device struct to give the subsystems a clue as to which domains they were allowed to use. I recognize that there isn't a one-size-fits-all solution to this problem so I'm open to different ideas. Jordan -- The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
