linux-kernel - Re: [PATCH 2/2] [RFC] base: soc: Allow early registration of a single SoC device

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Date:   Mon, 13 Mar 2017 13:46:16 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] [RFC] base: soc: Allow early registration of a single
 SoC device

Hi Arnd,

On Mon, Mar 13, 2017 at 1:41 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Thu, Mar 9, 2017 at 7:18 PM, Geert Uytterhoeven
> <geert+renesas@...der.be> wrote:
>> commit 1da1b3628df34a2a ("base: soc: Early register bus when needed")
>> added support for early registration of SoC devices from a
>> core_initcall().  However, some drivers need to check the SoC revision
>> from an early_initcall(), which is even earlier.
>>
>> While registering the SoC bus and device, and using soc_device_match(),
>> from an early_initcall() do work, the "soc" directory and the "soc0"
>> file end up wrongly in the sysfs root, as the "bus" resp. "devices"
>> directories haven't been created yet.
>>
>> To fix this, allow to register a single SoC device early on.
>> As long as the SoC bus isn't registered, soc_device_match() just
>> matches against this early device.
>> When the SoC bus is registered later, the early device is registered for
>> real.
>>
>> Note that soc_device_register() returns NULL (no error, but also not a
>> valid pointer) when registering an early device.  Hence platform devices
>> cannot be instantiated as children of the "soc0" node representing an
>> early SoC device.  This should not be an issue, as that practice has
>> been deprecated for new platforms.
>>
>> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>> ---
>> Is there a better way to fix this?
>> While I believe all current users register a single SoC device, we may
>> want to register multiple SoC devices in the future?
>>
>> soc_device_match() itself is completely independent from sysfs, but the
>> sysfs operations are tied intimately to the driver core.
>>
>> The "bus" and "devices" directories are created from do_basic_setup() ->
>> driver_init() -> devices_init() / buses_init(), which runs in between
>> do_pre_smp_initcalls() (early_initcall()) and do_initcalls().
>
> I'd prefer to not have to do the early registration at all and have fewer
> special cases. Can you list a specific example that requires this?

The specific example is the Renesas R-Car SYSC driver, which manages PM
Domains and thus needs to be initialized from an early_initcall.

> If we want to do early registration, then your implementation seems
> fine to me.

Thanks!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds