linux-kernel - Re: broken probe deferred for power-domains

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Date:   Wed, 27 Feb 2019 17:32:14 -0600
From:   Rob Herring <robh@...nel.org>
To:     Niklas Cassel <niklas.cassel@...aro.org>
Cc:     Alexander Graf <agraf@...e.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>
Subject: Re: broken probe deferred for power-domains

On Tue, Feb 26, 2019 at 8:18 PM Niklas Cassel <niklas.cassel@...aro.org> wrote:
>
> Hello Rob,
>
> Your patch e01afc325025 ("PM / Domains: Stop deferring probe
> at the end of initcall") breaks deferred probe for power domains.

With all the dependencies built-in, right?

What board in case I have one?

> The patch looks like this:
>
> +++ b/drivers/base/power/domain.c
> @@ -2253,7 +2253,7 @@ static int __genpd_dev_pm_attach(struct device *dev, struct device_node *np,
>                 mutex_unlock(&gpd_list_lock);
>                 dev_dbg(dev, "%s() failed to find PM domain: %ld\n",
>                         __func__, PTR_ERR(pd));
> -               return -EPROBE_DEFER;
> +               return driver_deferred_probe_check_state(dev);
>         }
>
>
> Having two drivers (both using module_platform_driver),
> one being a PD provider and one being a PD consumer.
>
> Before your patch:
> The PD consumer driver calls dev_pm_domain_attach(),
> and gets EPROBE_DEFER until the PD provider driver
> has been probed successfully.
>
> (The PD provider driver needs some regulators, so it
> is only successfully probed after the regulator driver
> has been probed successfully.)
>
> Anyway, dev_pm_domain_attach() returned success after
> the some deferred probes.
>
>
> After your patch:
> dev_pm_domain_attach() return ENODEV,
> which comes from driver_deferred_probe_check_state().
> Since it returns ENODEV rather than EPROBE_DEFER,
> the PD consumer driver fails to probe.
>
>
> The problem is related to your other patch 25b4e70dcce9
> ("driver core: allow stopping deferred probe after init").
>
> driver_deferred_probe_check_state() returns ENODEV if
> initcalls_done == true.
>
> initcalls_done is set from late_initcall(deferred_probe_initcall),
> in drivers/base/dd.c:
>        driver_deferred_probe_trigger();
>        flush_work(&deferred_probe_work);
>        initcalls_done = true;
>
> This does not seem very robust, since
>
> #1 It does not handle the case where two drivers have been
> deferred (put in the deferred probe pending list),
> where additionally one of the drivers has to be probed
> before the other.
>
> (We would need to call driver_deferred_probe_trigger() + flush_work()
> at least twice to handle this.)
>
> #2 Since this code is run from late_initcall(),
> initcalls_done might get set before other drivers using late_initcall()
> have even had a chance to run.

IMO, we should not have drivers using late_initcall. We need some
level just to do things at the end of boot. The same fragility exists
with the clock and regulator disabling.

> I can imagine that a driver using late_initcall() + EPROBE_DEFER
> will absolutely not work with this code.
>
>
> This patch fixes #1, but not #2.
> However, I assume that even this change would not work if we have 3
> drivers, where each driver a > b > c has to be probed, in that order.
> (and all of them were placed in the deferred probe pending list).

I thought a successful probe would trigger a retry too. I need to look
at it again.

Maybe it would be more robust to re-trigger probe until the pending
list doesn't change. Then we could handle any length of dependencies.

>
> Suggestions and patches are welcome.
>
>
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index a823f469e53f..3443cb78be9b 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -288,7 +288,6 @@ static int deferred_probe_initcall(void)
>         driver_deferred_probe_trigger();
>         /* Sort as many dependencies as possible before exiting initcalls */
>         flush_work(&deferred_probe_work);
> -       initcalls_done = true;
>
>         /*
>          * Trigger deferred probe again, this time we won't defer anything
> @@ -297,6 +296,8 @@ static int deferred_probe_initcall(void)
>         driver_deferred_probe_trigger();
>         flush_work(&deferred_probe_work);
>
> +       initcalls_done = true;
> +
>         if (deferred_probe_timeout > 0) {
>                 schedule_delayed_work(&deferred_probe_timeout_work,
>                         deferred_probe_timeout * HZ);
>
>
>
> Kind regards,
> Niklas