driver core: Add state_synced sysfs file for devices that support it

This can be used to check if a device supports sync_state() callbacks
and therefore keeps resources left on by the bootloader enabled till all
its consumers have probed.

This can also be used to check if sync_state() has been called for a
device or whether it is still trying to keep resources enabled because
they were left enabled by the bootloader and all its consumers haven't
probed yet.

Signed-off-by: Saravana Kannan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/ABI/testing/sysfs-devices-state_synced

···
+What:		/sys/devices/.../state_synced
+Date:		May 2020
+Contact:	Saravana Kannan <[e-mail]>
+Description:
+		The /sys/devices/.../state_synced attribute is only present for
+		devices whose bus types or driver provides the .sync_state()
+		callback. The number read from it (0 or 1) reflects the value
+		of the device's 'state_synced' field. A value of 0 means the
+		.sync_state() callback hasn't been called yet. A value of 1
+		means the .sync_state() callback has been called.
+
+		Generally, if a device has sync_state() support and has some of
+		the resources it provides enabled at the time the kernel starts
+		(Eg: enabled by hardware reset or bootloader or anything that
+		run before the kernel starts), then it'll keep those resources
+		enabled and in a state that's compatible with the state they
+		were in at the start of the kernel. The device will stop doing
+		this only when the sync_state() callback has been called --
+		which happens only when all its consumer devices are registered
+		and have probed successfully. Resources that were left disabled
+		at the time the kernel starts are not affected or limited in
+		any way by sync_state() callbacks.
+
+

drivers/base/dd.c

···
 		driver_deferred_probe_trigger();
 }
 
+static ssize_t state_synced_show(struct device *dev,
+				 struct device_attribute *attr, char *buf)
+{
+	bool val;
+
+	device_lock(dev);
+	val = dev->state_synced;
+	device_unlock(dev);
+	return sprintf(buf, "%u\n", val);
+}
+static DEVICE_ATTR_RO(state_synced);
+
 static int really_probe(struct device *dev, struct device_driver *drv)
 {
 	int ret = -EPROBE_DEFER;
···
 		goto dev_groups_failed;
 	}
 
+	if (dev_has_sync_state(dev) &&
+	    device_create_file(dev, &dev_attr_state_synced)) {
+		dev_err(dev, "state_synced sysfs add failed\n");
+		goto dev_sysfs_state_synced_failed;
+	}
+
 	if (test_remove) {
 		test_remove = false;
 
+		device_remove_file(dev, &dev_attr_state_synced);
 		device_remove_groups(dev, drv->dev_groups);
 
 		if (dev->bus->remove)
···
 		 drv->bus->name, __func__, dev_name(dev), drv->name);
 	goto done;
 
+dev_sysfs_state_synced_failed:
+	device_remove_groups(dev, drv->dev_groups);
 dev_groups_failed:
 	if (dev->bus->remove)
 		dev->bus->remove(dev);
···
 
 		pm_runtime_put_sync(dev);
 
+		device_remove_file(dev, &dev_attr_state_synced);
 		device_remove_groups(dev, drv->dev_groups);
 
 		if (dev->bus && dev->bus->remove)