On 10/20/2015 8:46 AM, Russell King - ARM Linux wrote:

On Mon, Oct 19, 2015 at 06:21:40PM +0200, Geert Uytterhoeven wrote:

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Hi Russell,

On Mon, Oct 19, 2015 at 5:35 PM, Russell King - ARM Linux
[off-list ref] wrote:

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What you can do is print those devices which have failed to probe at
late_initcall() time - possibly augmenting that with reports from
subsystems showing what resources are not available, but that's only
a guide, because of the "it might or might not be in a kernel module"
problem.

Well, adding those reports would give you a changelog similar to the
one in this series...

I'm not sure about that, because what I was thinking of is adding
a flag which would be set at late_initcall() time prior to running
a final round of deferred device probing.

Which round is the final round?
That's the one which didn't manage to bind any new devices to drivers,
which is something you only know _after_ the round has been run.

So I think we need one extra round to handle this.

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This flag would then be used in a deferred_warn() printk function
which would normally be silent, but when this flag is set, it would
print the reason for the deferral - and this would replace (or be
added) to the subsystems and drivers which return -EPROBE_DEFER.

That has the effect of hiding all the deferrals up until just before
launching into userspace, which should then acomplish two things -
firstly, getting rid of the rather useless deferred messages up to
that point, and secondly printing the reason why the remaining
deferrals are happening.

That should be a small number of new lines plus a one-line change
in subsystems and drivers.

Apart from the extra round we probably can't get rid of, that sounds OK to me.

Something like this.  I haven't put a lot of effort into it to change all
the places which return an -EPROBE_DEFER, and it also looks like we need
some helpers to report when we have only an device_node (or should that
be fwnode?)  See the commented out of_warn_deferred() in
drivers/gpio/gpiolib-of.c.  Adding this stuff in the subsystems searching
for resources should make debugging why things are getting deferred easier.

We could make driver_deferred_probe_report something that can be
deactivated again after the last deferred probe run, and provide the
user with a knob that they can turn it back on again.

I've tried this out on two of my platforms, including forcing
driver_deferred_probe_report to be enabled, and I get exactly one
deferred probe, so not a particularly good test.

The patch won't apply as-is to mainline for all files; it's based on my
tree which has some 360 additional patches (which seems to be about
normal for my tree now.)

I like the concept (I have been thinking along similar lines lately).
But I think this might make the console messages more confusing than
they are now.  The problem is that debug, warn, and error messages
come from a somewhat random set of locations at the moment.  Some
come from the driver probe routines and some come from the subsystems
that the probe routines call.  So the patch is suppressing some
messages, but not others.

One thing that seemed pretty obvious from the patches is that the
current probe routines are somewhat inconsistent in terms of messages,
and that there is room for a set of best practices for messaging.  That
is on my long term wish list, but I'm not sure I'll ever chase after
those windmills.

A couple of specific comments below.

The places where dev_warn_deferred() replaces dev_dbg(), we lose the
ability to turn on debugging and observe the driver reporting the
specific reason the deferral is occurring. So it would be useful to
add an "else dev_dbg()" in dev_warn_deferred() to retain that capability.

Couldn't you put the "driver_deferred_probe_report = true" here?  And then
not add another round of probes.

 	/* Sort as many dependencies as possible before exiting initcalls */
 	flush_workqueue(deferred_wq);
+
+	/* Now one final round, reporting any devices that remain deferred */
+	driver_deferred_probe_report = true;
+	driver_deferred_probe_trigger();
+	/* Sort as many dependencies as possible before exiting initcalls */
+	flush_workqueue(deferred_wq);
+
 	return 0;
 }

< snip >

-Frank