linux-kernel - Re: [PATCH 1/4] iommu: Add iommu_device_group callback and iommu

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Date:	Thu, 1 Dec 2011 13:00:46 +1100
From:	David Gibson <dwg@....ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Chris Wright <chrisw@...hat.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	joerg.roedel@....com, dwmw2@...radead.org,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	agraf@...e.de, scottwood@...escale.com, B08248@...escale.com
Subject: Re: [PATCH 1/4] iommu: Add iommu_device_group callback and
 iommu_group sysfs entry

On Thu, Dec 01, 2011 at 12:50:07PM +1100, Benjamin Herrenschmidt wrote:
> 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.

Except that the iommu group structure is not supposed to be specific
to PCI.

> 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.
> 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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