Subject: [RFC PATCH 4/7] iommu: provide helper function to configure an IOMMU for an of master

From: [email-masked] (Laurent Pinchart)
Date: Tue, 02 Sep 2014 22:08:58 +0300

Hi Will,

On Tuesday 02 September 2014 12:03:40 Will Deacon wrote:
> On Tue, Sep 02, 2014 at 11:51:54AM +0100, Laurent Pinchart wrote:
> > On Friday 29 August 2014 16:54:27 Will Deacon wrote:

[snip]

> > > diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
> > > index f9209666157c..a7d3b3a13b83 100644
> > > --- a/drivers/iommu/of_iommu.c
> > > +++ b/drivers/iommu/of_iommu.c

[snip]

> > > @@ -90,6 +91,41 @@ int of_get_dma_window(struct device_node *dn, const
> > > char
> > > *prefix, int index, }
> > > 
> > >  EXPORT_SYMBOL_GPL(of_get_dma_window);
> > > 
> > > +int of_iommu_configure(struct device *dev)
> > > +{
> > > +	struct of_phandle_args iommu_spec;
> > > +	struct bus_type *bus = dev->bus;
> > > +	const struct iommu_ops *ops = bus->iommu_ops;
> > > +	int ret = -EINVAL, idx = 0;
> > > +
> > > +	if (!iommu_present(bus))
> > > +		return -ENODEV;
> > > +
> > > +	/*
> > > +	 * We don't currently walk up the tree looking for a parent IOMMU.
> > > +	 * See the `Notes:' section of
> > > +	 * Documentation/devicetree/bindings/iommu/iommu.txt
> > > +	 */
> > > +	while (!of_parse_phandle_with_args(dev->of_node, "iommus",
> > > +					   "#iommu-cells", idx,
> > > +					   &iommu_spec)) {
> > > +		void *data = of_iommu_get_data(iommu_spec.np);
> > > +
> > > +		of_node_put(iommu_spec.np);
> > > +		if (!ops->add_device_master_ids)
> > > +			return -ENODEV;
> > 
> > Can't you move this outside of the loop ?
> 
> Yup, done.
> 
> > > +
> > > +		ret = ops->add_device_master_ids(dev, iommu_spec.args_count,
> > > +						 iommu_spec.args, data);
> > 
> > I'm curious, how do you envision support for devices connected to multiple
> > IOMMUs ? IOMMU drivers usually allocate a per-device archdata structure in
> > the add_device operation and store it in the dev->archdata.iommu field.
> > How would that work with multiple invocations of add_device or
> > add_device_master_ids ?
>
> Even devices with a single IOMMU could have multiple callbacks, since the
> IOMMU gets called back once per ID in reality. That means the IOMMU drivers
> will need to be tolerant of adding a master they already know about (by
> looking up the device and adding the additional IDs).

That wouldn't be too difficult to implement in IOMMU drivers, but I have a 
feeling we would duplicate the same logic in several drivers. Adding fields to 
struct iommu (possibly with a couple of helper functions) as mentioned below 
might help. Another option would be to call the add_device_master_ids a single 
time with a list of all ids, but the core code might become pretty ugly.

> Once I've reworked the data argument to be a struct iommu, we can add fields
> there if they're generally useful.

-- 
Regards,

Laurent Pinchart