linux-kernel - Re: [PATCH v5 04/11] clocksource/drivers: Add HPE GXP timer

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Message-ID: <CAK8P3a2ie8kFYstCYr6FO6+yFw7VxyJjWYyy9b+rUHu_u0YXPg@mail.gmail.com>
Date:   Tue, 26 Apr 2022 08:00:20 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "Hawkins, Nick" <nick.hawkins@....com>,
        "Verdun, Jean-Marie" <verdun@....com>,
        Joel Stanley <joel@....id.au>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 04/11] clocksource/drivers: Add HPE GXP timer

On Mon, Apr 25, 2022 at 10:38 PM Linus Walleij <linus.walleij@...aro.org> wrote:
> On Fri, Apr 22, 2022 at 3:16 PM Arnd Bergmann <arnd@...db.de> wrote:
> > On Thu, Apr 21, 2022 at 9:21 PM <nick.hawkins@....com> wrote:
> >
> > > +
> > > +static struct platform_device gxp_watchdog_device = {
> > > +       .name = "gxp-wdt",
> > > +       .id = -1,
> > > +};
> > > +/*
> > > + * This probe gets called after the timer is already up and running. This will create
> > > + * the watchdog device as a child since the registers are shared.
> > > + */
> > > +
> > > +static int gxp_timer_probe(struct platform_device *pdev)
> > > +{
> > > +       struct device *dev = &pdev->dev;
> > > +
> > > +       /* Pass the base address (counter) as platform data and nothing else */
> > > +       gxp_watchdog_device.dev.platform_data = local_gxp_timer->counter;
> > > +       gxp_watchdog_device.dev.parent = dev;
> > > +       return platform_device_register(&gxp_watchdog_device);
> > > +}
> >
> > I don't understand what this is about: the device should be created from
> > DT, not defined statically in the code. There are multiple ways of creating
> > a platform_device from a DT node, or you can allocate one here, but static
> > definitions are generally a mistake.
> >
> > I see that you copied this from the ixp4xx driver, so I think we should fix this
> > there as well.
>
> The ixp4xx driver looks like that because the register range used for
> the timer and the watchdog is combined, i.e. it is a single IP block:
>
>                 timer@...05000 {
>                         compatible = "intel,ixp4xx-timer";
>                         reg = <0xc8005000 0x100>;
>                         interrupts = <5 IRQ_TYPE_LEVEL_HIGH>;
>                 };
>
> Device tree probing does not allow two devices to probe from the same
> DT node, so this was solved by letting the (less important) watchdog
> be spawn as a platform device from the timer.
>
> I don't know if double-probing for the same register range can be fixed,
> but I was assuming that the one-compatible-to-one-driver assumption
> was pretty hard-coded into the abstractions. Maybe it isn't?

Having a child device is fine, my objection was about the way
the device is created from a 'static platform_device ...' definition
rather than having the device structure allocated at probe time.

> Another way is of course to introduce an MFD. That becomes
> problematic in another way: MFD abstractions are supposed to
> be inbetween the resource and the devices it spawns, and with
> timers/clocksources this creates a horrible special-casing since the
> MFD bus (the parent may be providing e.g. an MMIO regmap)
> then need to be early-populated and searched by the timer core
> from TIMER_OF_DECLARE() early in boot.
>
> So this solution was the lesser evil that I could think about.

There are multiple ways of doing this that we already discussed
in the thread. The easiest is probably to have a child node without
custom registers in the DT and then use the DT helpers to
populate the linux devices with the correct data.

       Arnd