Re: [PATCH v6 6/8] dma-mapping: detect and configure IOMMU in of_dma_configure

[Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>]

On Wednesday 17 December 2014 17:17:52 Will Deacon wrote:
> > > > I would hope that PCI is the only case we need to worry about for a while.
> > > > This means we just need to come up with another property or a set of properties
> > > > that we can put into a PCI host controller device node in order to describe
> > > > the mapping. These properties could be iommu-specific, so we add something
> > > > to the PCI core that calls a new iommu callback function that takes the
> > > > device node of the PCI host and the bus/device/function number as inputs.
> > > > 
> > > > In arm_setup_iommu_dma_ops(), we can then do something like
> > > > 
> > > >   if (dev_is_pci(dev)) {
> > > >           struct pci_dev *pdev = to_pci_dev(dev);
> > > >           struct device_node *node;
> > > >           unsigned int bdf;
> > > > 
> > > >           node = find_pci_host_bridge(pdev->bus)->parent->of_node;
> > > >           bdf = PCI_DEVID(pdev->bus->number, dev->devfn);
> > > > 
> > > >           iommu_setup_pci_dev(pdev, node, bdf);
> > > >   }
> > > 
> > > The other way to do this is have the IOMMU driver check dev_is_pci(dev)
> > > in add_device, then call an of_xlate_pci_bdf library function which could
> > > do the translation on-demand.
> > 
> > We'd still need to find the device node for the host controller in
> > common code, otherwise we don't have an of_xlate function to call.
> 
> I guess I was hoping that the translation code could be generic. I don't
> really see anything special about adding a constant to a magic number to
> obtain another magic number.
> 

If that's all we need, that's fine.

I was fearing that we'd get different host controllers using different
parts of the bdf number. E.g. one might pass down just bus/device
while another uses bus/device/function, so we'd need a shift and
an offset.

As part of the AMD review I found out that their SMMU implementation
only has 15 bits of address space, while bdf is 16 bits, so they
cut off the top bit and get aliasing between bus 0+n and bus 128+n.

Another SoC might have more aliasing if they have multiple domains.


	Arnd