Date:	Wed, 24 Jun 2015 02:14:30 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:	Rob Herring <robherring2@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Alexander Holler <holler@...oftware.de>,
	Alexandre Courbot <gnurou@...il.com>,
	Andrzej Hajda <a.hajda@...sung.com>,
	Arnd Bergmann <arnd@...db.de>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Grant Likely <grant.likely@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Kumar Gala <galak@...eaurora.org>, Len Brown <lenb@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Lv Zheng <lv.zheng@...el.com>, Mark Brown <broonie@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Robert Moore <robert.moore@...el.com>,
	Rob Herring <robh+dt@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	Stephen Warren <swarren@...dotorg.org>,
	Terje Bergström <tbergstrom@...dia.com>,
	Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH 02/13] driver-core: defer all probes until late_initcall

On Tuesday, June 23, 2015 04:37:57 PM Tomeu Vizoso wrote:
> On 23 June 2015 at 16:51, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > On Tuesday, June 23, 2015 04:17:29 PM Tomeu Vizoso wrote:
> >> On 23 June 2015 at 16:37, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >> > On Monday, June 22, 2015 07:07:08 PM Rob Herring wrote:
> >> >> On Fri, Jun 19, 2015 at 6:20 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >> >> > On Friday, June 19, 2015 03:36:46 PM Tomeu Vizoso wrote:
> >> >> >> On 18 June 2015 at 23:50, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >> >> >> > On Wednesday, June 17, 2015 03:42:12 PM Tomeu Vizoso wrote:
> >> >> >> >> To decrease the chances of devices deferring their probes because of
> >> >> >> >> dependencies not having probed yet because of their drivers not having
> >> >> >> >> registered yet, delay all probing until the late initcall level.
> >> >> >> >>
> >> >> >> >> This will allow us to avoid deferred probes completely later by probing
> >> >> >> >> dependencies on demand, or by probing them in dependency order.
> >> >> >> >>
> >> >> >> >> Signed-off-by: Tomeu Vizoso <tomeu.vizoso@...labora.com>
> >> >> >> >> ---
> >> >> >> >>  drivers/base/dd.c | 8 +++++++-
> >> >> >> >>  1 file changed, 7 insertions(+), 1 deletion(-)
> >> >> >> >>
> >> >> >> >> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> >> >> >> >> index a638bbb..18438aa 100644
> >> >> >> >> --- a/drivers/base/dd.c
> >> >> >> >> +++ b/drivers/base/dd.c
> >> >> >> >> @@ -407,6 +407,12 @@ int driver_probe_device(struct device_driver *drv, struct device *dev)
> >> >> >> >>       if (!device_is_registered(dev))
> >> >> >> >>               return -ENODEV;
> >> >> >> >>
> >> >> >> >> +     /* Defer all probes until we start processing the queue */
> >> >> >> >> +     if (!driver_deferred_probe_enable) {
> >> >> >> >> +             driver_deferred_probe_add(dev);
> >> >> >> >
> >> >> >> > Do I think correctly that this will effectively force everybody to use deferred
> >> >> >> > probing?
> >> >> >>
> >> >> >> Guess it depends on the meaning of "using deferred probing". It will
> >> >> >> defer the probe of the first device to late_initcall (which will
> >> >> >> happen much earlier in time than before), but afterwards all built-in
> >> >> >> drivers will be available and depending on the order in which we try
> >> >> >> to probe devices, none may actually ask to defer its probe.
> >> >> >
> >> >> > So this will break things like the PNP system driver which relies on probing
> >> >> > stuff at the fs_initcall stage for correctness.  It may also break other
> >> >> > things with similar assumptions.
> >> >>
> >> >> Yes, but I think that this can be done for only OF based devices
> >> >> rather than globally for all platform devices and solve that problem.
> >> >> Matching is already dependent of the type of device.
> >> >
> >> > Well, the current patch is not OF-only, though.
> >>
> >> Yeah, I'm currently looking at only delaying probing of devices
> >> created from OF data.
> >
> > I'm not sure if tying it hard to OF is not too restrictive.
> >
> > Maybe we can use some general opt-in mechanism that OF will just always use?
> 
> Would it help if buses called fwnode_driver_match_device() instead of
> the existing OF and ACPI variants and we did it in there?

Probably it would, but I'd need to see the actual patch. :-)

> I'm still not sure of how fwnode is used in machines with ACPI.

I'm not sure what you mean.  On ACPI systems struct fwnode_handle is embedded
in struct acpi_device and there is a pointer from struct device to that field
in the companion ACPI device object.

> But that would be quite a bit of work that I think should be left for
> a later series because otherwise this one is going to balloon in size
> really quickly.

Well, I'd prefer not to leave anything to a "later series" that may never be
submitted ...

> > In fact, we have a similar problem in ACPI where we have the _DEP object which
> > is used by firmware to describe dependencies between devices.
> 
> I would expect that classes/subsystems would be able to use that data
> in their class.get_dependencies() callback, if the passed fwnode is a
> ACPI node.

Yes, something like that.

But the point is that this really isn't OF-specific.

Thanks,
Rafael

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