linux-kernel - Re: [RFC PATCH 6/7] arm64: kernel: Apple CPU start driver

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Date:   Thu, 21 Jan 2021 12:14:24 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Mohamed Mediouni <mohamed.mediouni@...amail.com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.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>,
        Stan Skowronek <stan@...ellium.com>
Subject: Re: [RFC PATCH 6/7] arm64: kernel: Apple CPU start driver

On Wed, Jan 20, 2021 at 2:27 PM Mohamed Mediouni
<mohamed.mediouni@...amail.com> wrote:
> diff --git a/Documentation/devicetree/bindings/arm/cpus.yaml b/Documentation/devicetree/bindings/arm/cpus.yaml
> index 14cd727d3c4b..a6ff8cb3db1e 100644
> --- a/Documentation/devicetree/bindings/arm/cpus.yaml
> +++ b/Documentation/devicetree/bindings/arm/cpus.yaml
> @@ -176,6 +176,7 @@ properties:
>      oneOf:
>        # On ARM v8 64-bit this property is required
>        - enum:
> +          - apple
>            - psci
>            - spin-table
>        # On ARM 32-bit systems this property is optional

This uses a very generic identifier for doing something that may
be very specific to a particular SoC generation. It's going to be hard
to decide what the right abstraction will be for long-term maintenance,
so I'd recommend starting with a boot loader that implements
spin-table for secondary startup, and getting back to this after more
of the basic stuff works.

> +static int cpu_apple_start_prepare(unsigned int cpu)
> +{
> +       struct device_node *node;
> +       struct cpu_apple_start_info *info;
> +
> +       info = per_cpu_ptr(&cpu_apple_start_info, cpu);
> +
> +       if (info->pmgr_start && info->cputrc_rvbar && info->dbg_unlock)
> +               return 0;
> +
> +       node = of_find_compatible_node(NULL, NULL, "apple,startcpu");
> +       if (!node) {
> +               pr_err("%s: missing startcpu node in device tree.\n", __func__);
> +               return -EINVAL;
> +       }

Where is the binding documentation for this? The way you do a separate
of_iomap() for each CPU suggests that this is not a great binding to
start with. Are these perhaps just individual registers within a larger IP
block in the end?

      Arnd