[edk2-devel] [PATCH v3 3/8] DynamicTablesPkg: AcpiSsdtPcieLibArm: Fix _PRT description

[PierreGondois]

Hi,

On 1/29/22 7:20 PM, Marc Zyngier wrote:
> On Sat, 29 Jan 2022 15:52:02 +0000,
> Ard Biesheuvel <ardb at kernel.org> wrote:
>>
>> (+ Marc)
>>
>> On Fri, 28 Jan 2022 at 16:41, <Pierre.Gondois at arm.com> wrote:
>>>
>>> From: Pierre Gondois <Pierre.Gondois at arm.com>
>>>
>>> In ACPI 6.4, s6.2.13, _PRT objects describing PCI legacy interrupts
>>> can be defined following 2 models.
>>> In the first model, a _SRS object must be described to modify the PCI
>>> interrupt. The _CRS and _PRS object allows to describe the PCI
>>> interrupt (level/edge triggered, active high/low).
>>> In the second model, the PCI interrupt cannot be described with a
>>> similar granularity. PCI interrupts are by default level triggered,
>>> active low.
>>>
>>> GicV2 SPI interrupts are level or edge triggered, active high. To
>>> correctly describe PCI interrupts, the first model is used, even though
>>> Arm Base Boot Requirements v1.0 requires to use the second mode.
>>>
>>
>> There are two different issues here:
>>
>> - using separate 'interrupt link' device objects with an Interrupt()
>> resource rather than a simple GSIV number
>> - whether _PRS and _SRS need to be implemented on those link objects.
>>
>> The latter is simply not true - _PRS and _SRS are optional, and
>> pointless if there is only a single possible value, so there is really
>> no need to add them here.
>>
>> As for the choice between the link object or the GSIV number: I don't
>> think INTx interrupts on ARM systems are actually level low, and the
>> GSIV option is widely used, also in platforms that exist in
>> edk2-platforms, without any reported issues.
>>
>> I've cc'ed Marc, perhaps he can shed some light on this, but I'd
>> prefer to stick to the GSIV approach if we can, as it is much simpler.
> 
> I don't immediately see the point either. Yes, the GIC only supports
> level-high interrupts. However, all the PCIe implementations connected
> to it are inverting the level. If they don't, that's even simpler (the
> HW is broken).
> 
> Is this to address an apparent disconnect with the spec?
> 
> [...]
> 

1. _PRS/_SRS methods
I agree they are optional and meaningless here as interrupts are not dynamically configurable.
However linux is checking that:
- the interrupt used in _CRS is one of the possible interrupts advertised in _PRS. If not, then a warning is issued and the interrupt is not used.
- the _SRS method is present. If not, setting the interrupt fails.
If _PRS and _SRS method are really optional, it seems these checks should not happen.
For now, using a link object without _PRS/_SRS doesn't work.

Note:
The first check was initially done because an invalid interrupt was advertised in _CRS when valid interrupts were available in _PRS.
https://bugzilla.kernel.org/show_bug.cgi?id=2665

2. GSIV vs link object
The fist motivation was to accurately describe the interrupts.
Even though GIC interrupts must be active high, PCI interrupts are active low by default (according to spec), and the GSIV model doesn't allow to describe the polarity/activation state.

Another point that came out is that in linux, GSIV interrupts for PCI are configured as level triggered by default.
 From "Base System Architecture 1.0", sE.4 and sE.6, PCI interrupts can be level or edge triggered. More specifically, KvmTool configures PCI interrupts as edge triggered.
So the only way to describe an edge interrupt is to use a link object.


>>> -  The _PRS and _SRS are not supported, cf Arm Base Boot Requirements v1.0:
>>> +  Warning: The Arm Base Boot Requirements v1.0 states the following:
>>>     "The _PRS (Possible Resource Settings) and _SRS (Set Resource Settings)
>>>     are not supported."
>>> +  However, the first model to describe PCI legacy interrupts is used (cf. ACPI
>>> +  6.4 s6.2.13) as PCI defaults (Level Triggered, Active Low) are not compatible
>>> +  with GICv2 (must be Active High).
> 
> nit: GICv3 has the exact same behaviour. Why is v2 singled out?

No specific reason, GICv3 should have been in the comment aswell.

> 
> Thanks,
> 
> 	M.
> 

Regards,
Pierre


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