Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Content-Type: text/plain;
        charset=utf-8
Mime-Version: 1.0 (Mac OS X Mail 14.0 \(3654.60.0.2.5\))
Subject: Re: [RFC PATCH 7/7] irqchip/apple-aic: add SMP support to the Apple
 AIC driver.
From:   Mohamed Mediouni <mohamed.mediouni@caramail.com>
In-Reply-To: <CAL_Jsq+V74tMGJ==OVdsssfqTTa3TrFujfRjj0174uZNonpQSA@mail.gmail.com>
Date:   Thu, 21 Jan 2021 19:08:54 +0100
Cc:     Arnd Bergmann <arnd@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Hector Martin <marcan@marcan.st>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Stan Skowronek <stan@corellium.com>
Content-Transfer-Encoding: quoted-printable
Message-Id: <950D140B-491A-40EB-9FDB-D7173B86737B@caramail.com>
References: <20210120132717.395873-1-mohamed.mediouni@caramail.com>
 <20210120132717.395873-8-mohamed.mediouni@caramail.com>
 <CAK8P3a1qeVxTxZXpfMe70zpPCSBrTOz23ZTR=PHgw0PP9GUvbw@mail.gmail.com>
 <E77005CF-8B2A-4D17-9330-72ECFD7F3C93@caramail.com>
 <CAL_JsqJq_wfLrJm8rwspkgz81Be2V9WTudRjMAXSrZD50ewFYA@mail.gmail.com>
 <FEDB12D6-2916-4430-A8DC-FBD9352F3730@caramail.com>
 <CAL_Jsq+V74tMGJ==OVdsssfqTTa3TrFujfRjj0174uZNonpQSA@mail.gmail.com>
To:     Rob Herring <robh@kernel.org>
Precedence: bulk


> On 21 Jan 2021, at 18:37, Rob Herring <robh@kernel.org> wrote:
>=20
> On Thu, Jan 21, 2021 at 10:43 AM Mohamed Mediouni
> <mohamed.mediouni@caramail.com> wrote:
>>> On 21 Jan 2021, at 17:40, Rob Herring <robh@kernel.org> wrote:
>>> On Thu, Jan 21, 2021 at 6:52 AM Mohamed Mediouni
>>> <mohamed.mediouni@caramail.com> wrote:
>>>>> On 21 Jan 2021, at 13:44, Arnd Bergmann <arnd@kernel.org> wrote:
>>>>> On Wed, Jan 20, 2021 at 2:27 PM Mohamed Mediouni
>>>>> <mohamed.mediouni@caramail.com> wrote:
>=20
> [...]
>=20
>>>>>> @@ -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;
>>>>>>=20
>>>>>> +       aic.fast_ipi =3D of_property_read_bool(node, "fast-ipi");
>>>>>=20
>>>>> Where is this property documented, and what decides which one to =
use?
>>>> It=E2=80=99s getting documented in the next patch set.
>>>>=20
>>>> This property is there to enable support for older iPhone =
processors
>>>> later on, some of which do not have fast IPI support.
>>>>=20
>>>> On Apple M1, fast-ipi is always on.
>>>=20
>>> This should be implied by the compatible string which needs to be =
more
>>> specific and include the SoC name.
>>>=20
>>> Rob
>>=20
>> Then we=E2=80=99ll 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).
>>=20
>> Does that look right?
>=20
> If we did this from the start, it would evolve like this:
>=20
> 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"
>=20
> A11: "AAPL,a11-aic", "AAPL,a7-aic"
>=20
> 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:
>=20

The A11 AIC indeed can be used by older drivers that aren=E2=80=99t =
aware
of the fast IPI path introduced on A11 just fine.

> A11: "AAPL,a11-aic"
>=20
> If the A11 is different and not backwards compatible.
>=20
> Then M1 could be:
>=20
> M1: "AAPL,m1-aic", "AAPL,a11-aic"
>=20
> Or to even support an OS with only v1 support:
>=20
> M1: "AAPL,m1-aic", "AAPL,a11-aic", "AAPL,a7-aic"
>=20
> You don't really need the fallback here because there isn't any
> existing OS support and the baseline is the M1.
>=20
> 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.
>=20
> 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=E2=80=99t.
>=20
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=E2=80=99d be needed if ever in the future, but =
not uselessly
add entries to the driver that will not be currently used)

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=E2=80=A6 as the =
internal scheme
reveals that M1 could as well have been an A14X.

And there=E2=80=99s 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=E2=80=A6 with the marketing name being =
nowhere in their device
trees.

Thank you,

> Rob