Date:   Thu, 21 Jan 2021 12:57:34 -0600
From:   Rob Herring <robh@...nel.org>
To:     Mohamed Mediouni <mohamed.mediouni@...amail.com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Hector Martin <marcan@...can.st>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Stan Skowronek <stan@...ellium.com>
Subject: Re: [RFC PATCH 7/7] irqchip/apple-aic: add SMP support to the Apple
 AIC driver.

On Thu, Jan 21, 2021 at 12:09 PM Mohamed Mediouni
<mohamed.mediouni@...amail.com> wrote:
>
>
>
> > On 21 Jan 2021, at 18:37, Rob Herring <robh@...nel.org> wrote:
> >
> > On Thu, Jan 21, 2021 at 10:43 AM Mohamed Mediouni
> > <mohamed.mediouni@...amail.com> wrote:
> >>> On 21 Jan 2021, at 17:40, Rob Herring <robh@...nel.org> wrote:
> >>> On Thu, Jan 21, 2021 at 6:52 AM Mohamed Mediouni
> >>> <mohamed.mediouni@...amail.com> wrote:
> >>>>> On 21 Jan 2021, at 13:44, Arnd Bergmann <arnd@...nel.org> wrote:
> >>>>> On Wed, Jan 20, 2021 at 2:27 PM Mohamed Mediouni
> >>>>> <mohamed.mediouni@...amail.com> wrote:
> >
> > [...]
> >
> >>>>>> @@ -186,8 +325,11 @@ static int __init apple_aic_init(struct device_node *node,
> >>>>>>     if (WARN(!aic.base, "unable to map aic registers\n"))
> >>>>>>             return -EINVAL;
> >>>>>>
> >>>>>> +       aic.fast_ipi = of_property_read_bool(node, "fast-ipi");
> >>>>>
> >>>>> Where is this property documented, and what decides which one to use?
> >>>> It’s getting documented in the next patch set.
> >>>>
> >>>> This property is there to enable support for older iPhone processors
> >>>> later on, some of which do not have fast IPI support.
> >>>>
> >>>> On Apple M1, fast-ipi is always on.
> >>>
> >>> This should be implied by the compatible string which needs to be more
> >>> specific and include the SoC name.
> >>>
> >>> Rob
> >>
> >> Then we’ll eventually have two aic compatible strings, aic which is compatible
> >> with Apple A7 onwards and aicv2 which is a superset with fast IPI (introduced
> >> on the Apple A11, 3 years ago, with no further programmer-visible changes since
> >> then).
> >>
> >> Does that look right?
> >
> > If we did this from the start, it would evolve like this:
> >
> > A7: "AAPL,a7-aic"
> > A8: "AAPL,a8-aic", "AAPL,a7-aic"  # Read this as A8 AIC is backwards
> > compatible with A7 AIC
> > A9: "AAPL,a9-aic", "AAPL,a7-aic"
> >
> > A11: "AAPL,a11-aic", "AAPL,a7-aic"
> >
> > If the A11 version could work on an OS that only supported the
> > original model (sounds like this is the case) Or if it's not backwards
> > compatible:
> >
>
> The A11 AIC indeed can be used by older drivers that aren’t aware
> of the fast IPI path introduced on A11 just fine.
>
> > A11: "AAPL,a11-aic"
> >
> > If the A11 is different and not backwards compatible.
> >
> > Then M1 could be:
> >
> > M1: "AAPL,m1-aic", "AAPL,a11-aic"
> >
> > Or to even support an OS with only v1 support:
> >
> > M1: "AAPL,m1-aic", "AAPL,a11-aic", "AAPL,a7-aic"
> >
> > You don't really need the fallback here because there isn't any
> > existing OS support and the baseline is the M1.
> >
> > If you want to have generic fallback compatible strings with versions,
> > that's fine too. I'm not really a fan of version numbers that are just
> > made up by the binding author though. Most SoC vendors don't have
> > rigorous versioning of their IP and those that do seem to have a new
> > version on every SoC.
> >
> > The important part is *always* having an SoC specific compatible so
> > you can deal with any quirk or feature without having to change the
> > DTB. Everyone says blocks are 'the same' until they aren’t.
> >
> Is it fine if such a SoC-specific compatible is present but with having
> the driver only know about AAPL,a11-aic for example?
> (To just have it when it’d be needed if ever in the future, but not uselessly
> add entries to the driver that will not be currently used)

Yes, that's expected. You add the more specific compatible when you
add the feature or quirk work-around.

>
> On a tangent:
>
> The internal naming scheme used by Apple is off-by-one:
>
> Apple A14 for example is Apple H13P (H-series 13th gen processor, Phone)
> Apple M1 is Apple H13G (H-series 13th gen, G series)
> (And Apple A12X is Apple H11G for example, with A12 being H11P)
>
> Should we bother with those or use the marketing names? Especially because
> the beefier SoCs might not be of the H series anyway… as the internal scheme
> reveals that M1 could as well have been an A14X.
>
> And there’s also the other internal naming scheme:
> Apple A12 being t8020, Apple A12X being t8027
> Apple A14 being t8101
> Apple M1 being t8103
>
> T there means the foundry at which the chip was manufactured, in the cases above TSMC.
>
> Of course Apple itself uses both… with the marketing name being nowhere in their device
> trees.

I'd probably lean toward the marketing names, but don't really care as
long as you're consistent both for a given SoC and across generations.

Rob

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