On Tue, Oct 20, 2015 at 04:46:56PM +0100, Russell King - ARM Linux wrote:

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.

Yeah, plus I'd expect it to also result in better error reporting
overall if the subsystems are able to report when they fail to get
something rather than just returning an error to the driver.

+/**
+ * dev_warn_deferred() - report why a probe has been deferred
+ */
+void dev_warn_deferred(struct device *dev, const char *fmt, ...)
+{
+	if (driver_deferred_probe_report) {
+		struct va_format vaf;
+		va_list ap;
+
+		va_start(ap, fmt);
+		vaf.fmt = fmt;
+		vaf.va = ≈
+
+		dev_warn(dev, "deferring probe: %pV", &vaf);
+		va_end(ap);
+	}
+}
+EXPORT_SYMBOL_GPL(dev_warn_deferred);

I'm not currently able to think of a nice way of writing this but I think
what I'd really like to see from a driver point of view is something
which decays into dev_err() if it's a non-deferral error.  That way
drivers can have minimal log and return error handling code and we will
still get the output sensibly.  The best I can think of is something
like

   void dev_warn_deferred(struct device *dev, int err, const char *fmt, ...)

which requires the caller to pass in err twice to get it logged.  That's
not a thing of beauty but it gets the job done...  but perhaps your
original interface is better, it's a bit cleaner.