Re: [PATCH 0/4] iommu: Prevent oops in iommu_get() and while arch_iommu is in use

[David Cohen <dacohen@gmail.com>]

On Wed, Mar 30, 2011 at 4:56 PM, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
> On Wednesday 30 March 2011 15:50:37 Sakari Ailus wrote:
>> Laurent Pinchart wrote:
>> > Hi Sakari,
>>
>> Hi Laurent,
>>
>> > On Wednesday 30 March 2011 10:16:56 Sakari Ailus wrote:
>> >> Laurent Pinchart wrote:
>> >>> On Friday 25 March 2011 20:37:55 Ramirez Luna, Omar wrote:
>> >>>> On Fri, Mar 25, 2011 at 10:13 AM, Sakari Ailus wrote:
>> >>>>> Hi,
>> >>>>>
>> >>>>> This patchset is aimed to fix a problem in arch_iommu implementation
>> >>>>> references. When an actual arch_iommu implementation is not loaded
>> >>>>> while iommu_get() is being called results to a kernel oops, as well
>> >>>>> as removing an arch_iommu implementation which is in use.
>> >>>>
>> >>>> How about fixing the dependency instead? Right now iommu2 depends on
>> >>>> iommu because of the calls to
>> >>>> install_iommu_arch/uninstall_iommu_arch... we should change that
>> >>>> dependency to iommu depend on iommu2. Something like iommu (plat)
>> >>>> querying iommu2 (mach) for devices to install.
>> >>>
>> >>> The reason why iommu depends on iommu2 and not the other way around is
>> >>> because several mach-specific iommu implementations should be able to
>> >>> coexist in the same kernel. The right one should be loaded at runtime.
>> >>>
>> >>> I think that Sakari's patches correcty fix the problems he noticed.
>> >>> However, they won't fix one basic issue, which is that the iommu2
>> >>> module won't be automatically pulled in when the omap3isp module is
>> >>> loaded. The omap3isp driver will then fail to probe the device. That's
>> >>> better than crashing though.
>> >>
>> >> One option would be to specify the name of the module in the platform
>> >> data and request_module() that in omap_iommu_probe(). This would solve
>> >> the issue, not sure how pretty is this though.
>> >
>> > Do we need that ? My understanding is that a machine will need a single
>> > mach- specific iommu implementation only. Drivers shouldn't need to care
>> > about that.
>>
>> Well, no more than that there would have to be a driver for the IOMMU
>> for that very hardware.
>>
>> > The iommu implementation should be automatically selected based on the
>> > machine time.
>>
>> Machine type?
>>
>> I agree, but where is the selection made?
>
> The selection can be made by board code, or by the iommu implementations
> themselves if they're compiled in.

I prefer the first option. The second one will make the current
implementation be even more OMAP-only.
We have basically 3 layers:
IOVMM, IOMMU_GENERIC and IOMMU_SPECIFIC. The middle one should be
generic and don't care about machine types. The later one can be
handled by board code as it's machine specific and, for most of the
cases, I see no reason to let any other implementation besides the
machine type's to be loaded.

But the generic layer should not depend on any specific one. If
somebody decides to load the specific layer after the generic one, it
cannot be a problem.

Regards,

David

>
> --
> Regards,
>
> Laurent Pinchart
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