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    SubjectRe: [PATCH 02/10] Driver core: Add iommu_ops to bus_type
    On Wed, Sep 07, 2011 at 12:44:45PM -0700, Greg KH wrote:
    > On Wed, Sep 07, 2011 at 09:19:19PM +0200, Joerg Roedel wrote:
    > > Hi Greg,
    > >
    > > the bus_set_iommu() function will be called by the IOMMU driver. There
    > > can be different drivers for the same bus, depending on the hardware. On
    > > PCI for example, there can be the Intel or the AMD IOMMU driver that
    > > implement the iommu-api and that register for that bus.
    > Why are you pushing this down into the driver core? What other busses
    > becides PCI use/need this?

    Currently it is the platform_bus besides pci. The pci iommus are on x86
    and ia64 while all arm iommus use the platform_bus (by 'iommus' I only
    mean those implementing the iommu-api). Currently there are two drivers
    for arm iommus in /drivers/iommu.

    > If you can have a different IOMMU driver on the same bus, then wouldn't
    > this be a per-device thing instead of a per-bus thing?

    Well, I havn't seen a system yet where multiple iommus are on the same
    bus. Or to state it better, multiple iommus of different type that
    require different drivers. There is no 1-1 mapping between real hardware
    iommus and iommu_ops. There is only such a mapping for iommu drivers and
    iommu_ops. An iommu driver usually handles all hardware iommus of the
    same type in the system.

    So having iommu_ops per-device doesn't make much sense at this point.
    With this patch-set they are accessible by dev->bus->iommu_ops anyway.
    But if I am wrong on this I can change this of course.

    This patch-set improves the current situation where only on active
    iommu-driver is allowed to be active on a system (because of the global
    iommu_ops). But the main reason to put this into the bus_type structure
    is that it allows to generalize the device-handling on a bus between
    iommu drivers.

    > > On Wed, Sep 07, 2011 at 11:47:50AM -0700, Greg KH wrote:
    > > > > +#ifdef CONFIG_IOMMU_API
    > > > > +int bus_set_iommu(struct bus_type *bus, struct iommu_ops *ops)
    > > > > +{
    > > > > + if (bus->iommu_ops != NULL)
    > > > > + return -EBUSY;
    > > >
    > > > Busy?
    > >
    > > Yes, it signals to the IOMMU driver that another driver has already
    > > registered for that bus. In the previous register_iommu() interface this
    > > was just a BUG(), but I think returning an error to the caller is
    > > better. It can be turned back into a BUG() if it is considered better,
    > > though.
    > Can you ever have more than one IOMMU driver per bus? If so, this seems
    > wrong (see above.)

    As I said, I havn't seen such systems. But if they exist or are planned
    I am happy to redesign the whole thing.

    > > The IOMMUs are usually devices on the bus itself, so they are
    > > initialized after the bus is set up and the devices on it are
    > > populated. So the function can not be called on bus initialization
    > > because the IOMMU is not ready at this point.
    > Ok, that makes more sense, please state as much in the documentation.

    Will do, thanks.


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