Re: [RFC 1/3] iommu/arm-smmu: Add support to opt-in to stalling

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On Wed, Jan 11, 2017 at 4:36 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Tue, Jan 10, 2017 at 02:20:13PM -0500, Rob Clark wrote:
>> On Tue, Jan 10, 2017 at 12:52 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
>> > On Fri, Jan 06, 2017 at 11:26:49AM -0500, Rob Clark wrote:
>> >> Hmm, well we install the fault handler on the iommu_domain..  perhaps
>> >> maybe a combo of dts property (or deciding based on more specific
>> >> compat string), plus extra param passed in to
>> >> iommu_set_fault_hander().  The dts property or compat string to
>> >> indicate whether the iommu (and how it is wired up) can handle stalls,
>> >> and enable_stall param when fault handler is registered to indicate
>> >> whether the device itself can cope.. if either can't do stalling, then
>> >> don't set CFCFG.
>> >
>> > I thought about this some more, and I think you're right. Having
>> > iommu_set_fault_handler take a flags parameter indicating that, for example,
>> > the fault handler can deal with paging, is all we need to implement the
>> > per-master opt-in functionality for stalling faults. There's no real
>> > requirement to standardise a generic firmware property for that (but
>> > we still need *something* that says stalling is usable on the SMMU --
>> > perhaps just the compatible string is ok).
>>
>> btw, it occurred to me that maybe it should be flags param to
>> iommu_attach_device() (just in case fault handler not installed?)
>> otoh stalling without a fault handler is silly, but I guess we need it
>> to infer whether stalling can be supported by other devices on same
>> iommu.. tbh I'm on a bit shaky ground when it comes to multiple
>> devices per iommu since the SoC's I'm familiar with do it the other
>> way around.  But I guess you have thought more about the multi-device
>> case, so figured I should suggest it..
>
> I don't think it works at attach time, because the stalling property belongs
> to the domain, rather than the individual devices within it. Similarly, I
> don't think we should allow this property to be toggled once devices have
> been attached.
>

hmm, I was more thinking of cases where drivers for particular devices
need some work (ie. like potentially disabling hw hang detect during
faults).. I guess we could have three levels, that all have to be true
in order to enable stall: smmu, domain (pass flags in to
iommu_domain_alloc()??), and device (iommu_attach_device())?

BR,
-R
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