[PATCH v2 1/8] driver core: make deferring probe after init optional

[Rob Herring]

Deferred probe will currently wait forever on dependent devices to probe,
but sometimes a driver will never exist. It's also not always critical for
a driver to exist. Platforms can rely on default configuration from the
bootloader or reset defaults for things such as pinctrl and power domains.
This is often the case with initial platform support until various drivers
get enabled. There's at least 2 scenarios where deferred probe can render
a platform broken. Both involve using a DT which has more devices and
dependencies than the kernel supports. The 1st case is a driver may be
disabled in the kernel config. The 2nd case is the kernel version may
simply not have the dependent driver. This can happen if using a newer DT
(provided by firmware perhaps) with a stable kernel version.

Subsystems or drivers may opt-in to this behavior by calling
driver_deferred_probe_check_init_done() instead of just returning
-EPROBE_DEFER. They may use additional information from DT or kernel's
config to decide whether to continue to defer probe or not.

Cc: Alexander Graf <agraf@xxxxxxx>
Signed-off-by: Rob Herring <robh@xxxxxxxxxx>
---
 drivers/base/dd.c      | 17 +++++++++++++++++
 include/linux/device.h |  2 ++
 2 files changed, 19 insertions(+)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index c9f54089429b..d6034718da6f 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -226,6 +226,16 @@ void device_unblock_probing(void)
 	driver_deferred_probe_trigger();
 }
 
+int driver_deferred_probe_check_init_done(struct device *dev, bool optional)
+{
+	if (optional && initcalls_done) {
+		dev_WARN(dev, "ignoring dependency for device, assuming no driver");
+		return -ENODEV;
+	}
+
+	return -EPROBE_DEFER;
+}
+
 /**
  * deferred_probe_initcall() - Enable probing of deferred devices
  *
@@ -240,6 +250,13 @@ static int deferred_probe_initcall(void)
 	/* 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
+	 * that is optional
+	 */
+	driver_deferred_probe_trigger();
+	flush_work(&deferred_probe_work);
 	return 0;
 }
 late_initcall(deferred_probe_initcall);
diff --git a/include/linux/device.h b/include/linux/device.h
index 477956990f5e..f3dafd44c285 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -334,6 +334,8 @@ struct device *driver_find_device(struct device_driver *drv,
 				  struct device *start, void *data,
 				  int (*match)(struct device *dev, void *data));
 
+int driver_deferred_probe_check_init_done(struct device *dev, bool optional);
+
 /**
  * struct subsys_interface - interfaces to device functions
  * @name:       name of the device function
-- 
2.17.0