Stephen Warren <swarren@xxxxxxxxxxxxx> wrote @ Wed, 30 Oct 2013 22:58:58 +0100:
> On 10/25/2013 03:11 AM, Thierry Reding wrote:
> ...
> > So my proposed solution for the IOMMU case is to treat it the same
> > as any other resources. Perhaps resource isn't the right word, but
> > at the core the issue is the same. A device requires the services
> > of an IOMMU so that it can be put into the correct address space.
> > If the IOMMU is not available yet it cannot do that, so we simply
> > return -EPROBE_DEFER and cause the probe to be retried later.
>
> Personally, I view deferred probe as being used when one device
> requires either a resource /or/ a service provided by another, not
> /just/ when there's a resource dependency. Hence, I think it fits
> perfectly here.
>
> So I agree with Thierry: In other words, I think the solution is for
> all devices that are affected by an IOMMU to have a property such as:
>
> iommu = <&iommu_phandle iommu_specifier>;
Agree
> (and the DT node for the IOMMU will contain e.g. an #iommu-cells property)
As explained in another mail[1], "#iommu-cells" could vary, depending
on a device since it's arbitral number of arguments(IDs) right
now. This could be a fixed size of bitmap(64), 2 cells since we know
"64" would be enough in the future as well as below.
Now:
deviceA {
iommu = <&smmu 6>;
};
deviceB {
iommu = <&smmu 2 16>;
};
New:
smmu: iommu {
#iommu-cells = <2>;
};
deviceA {
iommu = <&smmu 0x00000000 0x00000040>;
};
deviceB {
iommu = <&smmu 0x00000000 0x00000104>;
};
But then we cannot use SWGROUP ID "macros" in DT files since DTC
cannot perse OR("|") operations as below. Or can we?
#define SWGROUP_ID_A BIT(2)
#define SWGROUP_ID_B BIT(16)
deviceB {
iommu = <&smmu SWGROUP_ID_A | SWGROUP_ID_B>;
};
So I thought that arbitral length of arguments may be easier to read as
below:
#define SWGROUP_ID_A 2
#define SWGROUP_ID_B 16
deviceB {
iommu = <&smmu SWGROUP_ID_A
SWGROUP_ID_B>;
};
> ... and for the driver to explicitly parse that property, and wait
> until the driver for iommu_phandle is ready. Exactly the same as any
> other resource/service dependency.
>
> That will solve all the problems.
>
> The only downside is that every driver needs to contain code to parse
> that property. However, I think that's just one function call; the
> actual implementation of that function can be unified somewhere inside
> core code in drivers/iommu/.
Yes, but only missing part now is that, we could do this with
"bus_notifier", but the current bus_notifier doesn't have the feature
to return error(-EPROBE_DEFER). This could be modified so that
bus_notifier could return (-EPROBE_DEFER) to postpone
probing. Alternatively this could be done in some core probe code as
well as Thierry pointed out.
[1] In the reply of "[PATCHv3 14/19] iommu/tegra: smmu: Get "nvidia,memory-clients" from DT"
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