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

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, April 7, 2022 1:17 AM
> 
> 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.

Intel discovers IOMMUs (and optionally ACPI namespace devices) from
ACPI, but there is no ACPI description for PCI devices i.e. the current
logic of device_get_dma_attr() cannot be used on PCI devices. 

> >
> > 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.

Probably not. If adding an iommu may change the coherency it would
come from specific IOPTE attributes for a device. The fact whether the
device is mapped by iommu doesn't tell that information.

> 
> 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.
> 

if going this way probably device_get_dma_attr() should be renamed to
device_fwnode_get_dma_attr() instead to make it clearer?

Thanks
Kevin
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