Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with dev_is_dma_coherent()

Jason Gunthorpe
2022-04-06T19:52:45Z

On Wed, Apr 06, 2022 at 06:10:31PM +0200, Christoph Hellwig wrote:
> On Wed, Apr 06, 2022 at 01:06:23PM -0300, Jason Gunthorpe wrote:
> > On Wed, Apr 06, 2022 at 05:50:56PM +0200, Christoph Hellwig wrote:
> > > On Wed, Apr 06, 2022 at 12:18:23PM -0300, Jason Gunthorpe wrote:
> > > > > Oh, I didn't know about device_get_dma_attr()..
> > >
> > > Which is completely broken for any non-OF, non-ACPI plaform.
> >
> > I saw that, but I spent some time searching and could not find an
> > iommu driver that would load independently of OF or ACPI. ie no IOMMU
> > platform drivers are created by board files. Things like Intel/AMD
> > discover only from ACPI, etc.
>
> s390?

Ah, I missed looking in s390, hyperv and virtio..

hyperv is not creating iommu_domains, just IRQ remapping

virtio is using OF

And s390 indeed doesn't obviously have OF or ACPI parts..

This seems like it would be consistent with other things:

enum dev_dma_attr device_get_dma_attr(struct device *dev)
{
const struct fwnode_handle *fwnode = dev_fwnode(dev);
struct acpi_device *adev = to_acpi_device_node(fwnode);

if (is_of_node(fwnode)) {
if (of_dma_is_coherent(to_of_node(fwnode)))
return DEV_DMA_COHERENT;
return DEV_DMA_NON_COHERENT;
} else if (adev) {
return acpi_get_dma_attr(adev);
}

/* Platform is always DMA coherent */
if (!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE) &&
!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU) &&
!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL) &&
device_iommu_mapped(dev))
return DEV_DMA_COHERENT;
return DEV_DMA_NOT_SUPPORTED;
}
EXPORT_SYMBOL_GPL(device_get_dma_attr);

ie s390 has no of or acpi but the entire platform is known DMA
coherent at config time so allow it. Not sure we need the
device_iommu_mapped() or not.

We could alternatively use existing device_get_dma_attr() as a default
with an iommu wrapper and push the exception down through the iommu
driver and s390 can override it.

Thanks,
Jason