Re: [PATCH 1/4] iommu: Add iommu_device_group callback and iommu_group sysfs entry

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2011-11-30 at 17:04 -0800, Chris Wright wrote:
> Heh.  Put it another way.  Generating the group ID is left up to the
> IOMMU.  This will break down when there's a system with multiple IOMMU's
> on the same bus_type that don't have any awareness of one another.  This
> is not the case for the existing series and x86 hw.
> 
> I'm not opposed to doing the allocation and ptr as id (taking care for
> possibility that PCI hotplug/unplug/replug could reuse the same memory
> for group id, however).  Just pointing out that the current system works
> as is, and there's some value in it's simplicity (overloading ID ==
> group structure + pretty printing ID in sysfs, for example). 

Well, ID can work even with multiple domains since we have domains
numbers. bdfn is 16-bit, which leaves 16-bit for the domain number,
which is sufficient.

So by encoding (domain << 16) | bdfn, we can get away with a 32-bit
number... it just sucks.

Note that on pseries, I wouldn't use bdfn anyway, I would use my
internal "PE#" which is also a number that I can constraint to 16-bits.

So I can work with a number as long as it's at least an unsigned int
(32-bit), but I think it somewhat sucks, and will impose gratuituous
number <-> structure conversions all over, but if we keep the whole
group thing an iommu specific data structure, then let's stick to the
number and move on with life.

We might get better results if we kept the number as

struct iommu_group_id {
	u16	domain;
	u16	group;
};

(Or a union of that with an unsigned int)

That way the domain information is available generically (can be match
with pci_domain_nr() for example), and sysfs can then be layed out as

/sys/bus/pci/groups/<domain>/<id>

Which is nicer than having enormous id's

Cheers,
Ben.