Re: [PATCH v3 2/2] irqchip/apple-aic: Add support for A7-A11 SoCs

[Nick Chan]

On 4/10/2022 23:56, Sven Peter wrote:
> Hi,
>
>
> On Tue, Oct 4, 2022, at 13:27, Konrad Dybcio wrote:
> > Add support for A7-A11 SoCs by if-ing out some features only present
> > on:
> >
> > * A11 & newer (implementation-defined IPI & UNCORE registers)
> > * A11[1] & newer (fast IPI support).
> >
> > UNCORE/UNCORE2 and IPI registers conveniently both first appeared on
> > A11, so introduce just one check for that.
> >
> > Knowing whether the SoC supports the latter is necessary, as they are
> > written to, even if fast IPI is disabled.
>
> AFAIK that's only an artifact in this driver: It was added to prevent an FIQ
> storm in case there were pending fast ipis (i.e. the bootloader was broken ;))
> when this driver didn't support fast ipis yet.
>
> > This in turn causes a crash
> > on older platforms, as the implemention-defined registers either do
> > something else or are not supposed to be touched - definitely not a
> > NOP though.
> >
> > [1] A11 is supposed to use this feature, but it currently doesn't work
> > for reasons unknown and hence remains disabled. It can easily be enabled
> > on A11 only, as there is a SoC-specific compatible in the DT with a
> > fallback to apple,aic. That said, it is not yet necessary, especially
> > with only one core up, and it has worked a-ok so far.
>
> Just to make sure I understand this correctly - we have the following three situations:
>
> - base: no fastipi, no uncore, will work on A11 and M1 though
> - A11: fastipi and uncore but fastipi is broken (possibly due to HW errata or some bug in this driver that only happens on A11)
> - M1 (or maybe even A12 already, doesn't matter though): fastipi and uncore support
>
> If we figured out _why_ fastipi is broken on A11 we would only need a single
> feature flag to enable both uncore and fastipi but for now we need two to
> disable fastipi for A11

The previous issues with fast IPI does not seem to be reproducible with thisversion of patches anymore, at least when modifying the device tree to useapple,t8103-aic the device still boots. Tested on iPhone X.
> I'm also curious: What are the symptoms when you enable fastipi on A11?
>
>
> Sven


Nick Chan