RE: [PATCH 02/10] Driver core: Add iommu_ops to bus_type

[Sethi Varun-B16395]

> -----Original Message-----
> From: kvm-owner@xxxxxxxxxxxxxxx [mailto:kvm-owner@xxxxxxxxxxxxxxx] On
> Behalf Of Joerg Roedel
> Sent: Thursday, September 08, 2011 12:49 AM
> To: Greg KH
> Cc: Joerg Roedel; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; Alex Williamson; Ohad
> Ben-Cohen; David Woodhouse; David Brown; kvm@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 02/10] Driver core: Add iommu_ops to bus_type
> 
> 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.
> 
> 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.
> 
> > > +
> > > +	bus->iommu_ops = ops;
> > > +
> > > +	/* Do IOMMU specific setup for this bus-type */
> > > +	iommu_bus_init(bus, ops);
> > > +
> > > +	return 0;
> > > +}
> > > +EXPORT_SYMBOL_GPL(bus_set_iommu);
> >
> > I don't understand what this function is for, and who would call it.
> 
> It is called by the IOMMU driver.
> 
> > Please provide kerneldoc that explains this.
> 
> Will do.
> 
> > > @@ -67,6 +68,9 @@ extern void bus_remove_file(struct bus_type *,
> struct bus_attribute *);
> > >   * @resume:	Called to bring a device on this bus out of sleep mode.
> > >   * @pm:		Power management operations of this bus, callback
> the specific
> > >   *		device driver's pm-ops.
> > > + * @iommu_ops   IOMMU specific operations for this bus, used to
> attach IOMMU
> > > + *              driver implementations to a bus and allow the driver
> to do
> > > + *              bus-specific setup
> >
> > So why is this just not set by the bus itself, making the above
> > function not needed at all?
> 
> 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.
Well, at what point would the add_device_group (referring to patch set posted by Alex) call back be invoked?

-Varun


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