Re: [RFC][PATCH] driver core: Extend returning EPROBE_DEFER for two minutes after late_initcall

From: John Stultz <john.stultz@linaro.org>
To: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: lkml <linux-kernel@vger.kernel.org>,
	Rob Herring <robh@kernel.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Kevin Hilman <khilman@kernel.org>,
	Ulf Hansson <ulf.hansson@linaro.org>, Pavel Machek <pavel@ucw.cz>,
	Len Brown <len.brown@intel.com>, Todd Kjos <tkjos@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Linux PM list <linux-pm@vger.kernel.org>
Subject: Re: [RFC][PATCH] driver core: Extend returning EPROBE_DEFER for two minutes after late_initcall
Date: Thu, 13 Feb 2020 20:05:05 -0800	[thread overview]
Message-ID: <CALAqxLWSXGQ0eD-1cxdB-mkJkyySRLo2MqZ4Y0YokhSZJJ6f-g@mail.gmail.com> (raw)
In-Reply-To: <20200214021922.GO1443@yoga>

On Thu, Feb 13, 2020 at 6:19 PM Bjorn Andersson
<bjorn.andersson@linaro.org> wrote:
> The purpose of 25b4e70dcce9 ("driver core: allow stopping deferred probe
> after init") is to ensure that when the kernel boots with a DeviceTree
> blob that references a resource (power-domain in this case) that either
> hasn't been compiled in, or simply doesn't exist yet, it should continue
> to boot - under the assumption that these resources probably aren't
> needed to provide a functional system.
>
> I don't think your patch maintains this behavior, because when userspace
> kicks in and load kernel modules during the first two minutes they will
> all end up in the probe deferral list. Past two minutes any event that
> registers a new driver (i.e. manual intervention) will kick of a new
> wave of probing, which will now continue as expected, ignoring any
> power-domains that is yet to be probed (either because they don't exist
> or they are further down the probe deferral list).

Hmm. I'll have to look at that again. I worry the logic is overloaded
a bit, because the logic in __driver_deferred_probe_check_state() will
only return -EPROBE_DEFER before late_initcall otherwise it returns
-ETIMEDOUT or 0.  So if we call__genpd_dev_pm_attach() after
late_initcall and the pd isn't ready, the driver probe will fail
permanently and not function.

I'd think in the case you describe (correct me if I'm misunderstanding
you), modules that load in the first two minutes would hit
EPROBE_DEFER only if a dependency is missing, and will continue to try
to probe next round. But once the two minutes are up, they will catch
ETIMEDOUT and fail permanently.

> You can improve the situation somewhat by calling
> driver_deferred_probe_trigger() in your
> deferred_initcall_done_work_func(), to remove the need for human
> intervention. But the outcome will still depend on the order in
> deferred_probe_active_list.

Ok. I'll take a look at that.

Thanks so much for the feedback!
-john